It's difficult, with your mother. I do understand - been there done that, with a range of family members. Part of the problem is that your mother undoubtedly loves you and loves your son (or wants to). But his behaviour is undoubtedly challenging. However, there are two possible reaons for your son being ab ehaviour problem:
1) He hasadevelopmental problem such as the autism you've had suggested; or
2) You're a bad mother.
Now, to your mother, the possibility tyhat her grandson could have something seriously wrong, is just too upsetting to think about. It's a lot easier to use denial and to instead insist that it's simply bad parenting. Much easier to believe her daughter is an incompetent parent, than her beloved grandson has problems.
You won't change her mind easily. I went through this especially with mother in law. I don't know how I would have gone with my own parents because they had both died before it became an issue. My mother was aware there were concerns but she died when difficult child 3 was only two years old, plus she was in hospital hundreds of miles away, she had only seen him a handful of times. She knew he was exceptional in a number of ways, I mentioned to her that difficult child 3 was very similar to a family member who was a noted musical genius in his youth. "Glory be, I hope not," was my mother's reply.
My father in law died not long afterwards. We were becoming aware that difficult child 3 had problems, but we were too tied up with father in law's health problems and the doctor was telling me to not worry about difficult child 3. It was only a few months after father in law's death, that we had the welfare authorities called on us (for alleged "emotional neglect" and failure to have his condition assessed - even though he was on waiting lists for assessment).
So mother in law was a new widow, which only made it harder for her to accept that ANY of her grandchildren had problems.
She needed to be kept occupied and involved, but it was difficult for me with her constant criticism of my parenting. She did try to curb herself, but I know her well now - when she is stressed or "off balance", she attacks other people. And this was upsetting her, that difficult child 3 was odd. She already knew that difficult child 1 was odd but that was just his individuality, in her mind.
But in the aftermath of the deaths in the family, mother in law had come to stay and as a result, had first-hand experience not only of her grandsons' odd behaviour, but also just how hard husband & I worked with the kids, to keep the behaviour consistent. She delighed in the obvious high intelligence and held onto that. But when the time came for difficult child 3 to have a thorough psychometric assessment as part of a multidiscplinary investigation, I invited mother in law to come along too. I didn't have my own mother for support; plus I felt mother in law needed to be kept in the loop, for her own understanding.
Not too surprisingly, mother in law was furious at the diagnosis of autism we were given that day. She hated the neuropsychologist testing process (she & I watched form a dark room on the other side of the one-way glass). mother in law contributed her own views and feelings about difficult child 3 (which were very useful, even in their inaccuracy).
I felt the outcome was satisfactory, however. I also felt there were flaws in ther assessment process but otherwise the diagnosis was correct. She & I spent a lot of time in the car on the way home, discussing the whole thing (including the diagnosis). She immeidately insisted I arrange for a second opinion, which I was happy to do (because I wanted it either confirmed for sure, or challenged if it was at all on shaky ground).
But over the next months & years, especially after the diagnosis was confirmed, mother in law would cut out articles on autism, would read them, discuss them with me. She would send copies off to her daughter (husband's sister). When husband's sister comes to visit mother in law and is then exposed to difficult child 3 (and the other kids) she has at times been critical of my parenting. But mother in law had by this time (over several years) become my champion in this. And I have since heard sis-in-law say things like, "I am amazed at how patient Marg is, with difficult child 3. Whatever she is doing, seems to work. I don't think I would have the patience but he is doing so much better every time we see him."
By involving mother in law in this process, I had intended to provide someone else for her to challenge - the doctor. Let her attack HIM and not my parenting... but as it turned out, it has also exposed her to the information she needed to take on board, in order to begin to understand.
I have enlisted her as an ally. At times I would rather not discuss things with her because she really doesn't like the idea of us seeing a therapist, for example (very old prejudice against the profession in general). But she's even coming around to this, since I explained to her how CBT works and how it's not "just psycho mumbo-jumbo".
It hasn't been easy to do it this way, but I feel it has been worth the effort. That's not to say there aren't still times when mother in law does the wrong thing (such as teasing difficult child 3, or chiding him for behaviour he can't control). But she's a lot better than she has been, or would be if I hadn't involved her so much.
Only you can decide if you can do this, or if it would work with your mother.
In some families, the mother still holds a position of Ultimate Authority and All knowledge is Vested in the Matriarch. mother in law is like this. Whereas with my own mother, although she was kept in the loop with a lot of things she never told us what to do unless we asked her opinion. She would always say, "It's up to you, dear." She also never insisted on being told what we weren't comfortable about sharing, whereas mother in law has been accustomed to being told every little detail (even when I have felt it to be a major invasion of my privacy). husband as the eldest, has never been able to keep secrets from her, not without a LOT of difficulty.
Families are always different. For some of us, our mothers feel they have a right to continue to control us. And we hand them that right when wwe continue to confide in them, every last little detail. Then we get upset when they keep hounding us with their opinion.
But the reason they do this - WE have just 'asked' them to do it, purely by involving them in the first place! We confide in them, then get upset when the advice they think we're asking for, hurts our feelings! WHat we need to do is have a frank and loving conversation. If you don't want the maternal criticism, you need to say to her (over a cup of coffee), "Mum, I love you and you will always be my mother. I know you love difficult child but he is MY son, I have to be the one to make the decisions concerning him. I also see a lot more of him than you do, and I am deeply concerned that there is something wrong. You taught me everything I know about being a mother, but I have to fly solo now. However, the parental instinct you bequeathed to me tells me we have problems. I need your support in this, I know it's painful. It hurts me too, to have to say there is something wrong. I would be the first to blame myself, but that won't get him any help. And if by chance it IS all my fauly, that it's just bad parenting - then having difficult child assessed NOW will find that out all the sooner. And whatever the problem - something seriously wrong, or just me being inept - the sooner we know, the sooner we can begin to fix it."
I knew mother in law would tell me what she thought always, as she does as is her habit. So by me involving her, I wasn't making any more trouble for myself. I was in fact hoping to reduce the heat from me (and it worked).
UNless she's a real witch (and maybe not even then) your mother loves you and despite the crassness of it, is speaking out of love. But you need to stop it happening because the last thing you need right now, is to have your resolve weakened and undermined by criticism that is misplaced.
Hang in there. If you can tame a difficult toddler, you can tame your own mother. And vice versa.
Marg