You need to understand that your son acts out more with you because he trusts your love. It stinks to be you, but it is true. Your son feels that he can let go and let his feelings out around you, and he cannot around SO. He trusts that you will love him anyway. I had this explained to me by more than a couple of therapists and doctors with my son. He was always worse with me than with anyone because I would love him no matter what. I would also stop him before he went so far as to hurt anyone else if I could. Well, anyone but me. It doesn't make it easier to handle in the moment, but understanding what is going on behind the behavior is a step toward making things better, at least it always was for us.
I think you NEED to get the child into Occupational Therapy for sensory integration disorder. If it is suspected that he has any form of autism, he has some degree of sensory integration disorder (Sensory Integration Disorder (SID)). This is when your brain does not process input from your senses the way a normal person's brain does. The world is too loud, or too quiet or too bright, or too dull, or too strongly flavored or the textures are wrong or slimy or off, any sense can be off. I have sensory issues and they don't ever go away, not at any age. You don't outgrow them. Learning how to handle them as a kid is a very good thing. There are things you can do to help, especially as young as your kids are. They are at the best ages to get them into Occupational Therapist (OT) for this.
First you need an evaluation. Insist that school do this, but you may also need a private evaluation. You will need to ask for the Occupational Therapist (OT) to teach you brushing therapy. If it is not offered, ask for it. A soft non soaped surgical scrub brush is used over the body in a certain order, usually followed by gentle joint compressions. It makes an incredible and amazing difference in a child. It is actually PROVEN to rewire a child's brain without any medication. It creates new pathways in the child's brain! It changes how the brain uses input from the senses. You MUST be taught how to do this by an Occupational Therapist, but it only takes on session to learn how. Then you do it at home.
The other aspects of therapy for Sensory Integration Disorder (SID) involve providing a sensory diet for your child. A well rounded sensory diet includes all the types of sensory input that your child's brain needs. The good part of this is that usually the types of input your child's brain needs are things your child will like. Your child will be drawn to these activities already for the most part. I laughed when they told me that my youngest child needed deep pressure on his head, like sitting on his head. He spent hours watching television upside down on the couch. He would look like he was sitting with his legs crossed tailor style, but instead of his legs on the couch, his head would be on the seat of the couch and his legs were in the air. We never even thought to fuss at him. He was our 3rd kid. He wasn't kicking anyone, or bothering anyone, so why would we tell him to stop? Turned out, it was good for him! Who knew?
Almost everyone with autism has some degree of this. The earlier you can get help for this, the more successful and high functioning your child can be. To learn more about this, read The Out of Sync Child by Kranowitz. To find activities that are fun and will really help your kids, get a copy of The Out of Sync Child Has Fun by Kranowitz. If you can only buy one book, get the first book from the library and the Has Fun book from the bookstore. My family went through several copies. The book truly is FUN!! Every kid in the neighborhood would come running when I would start an activity out of the book. Somehow they knew and they all appeared. Luckily there are tips to make the activities less expensive throughout the book. We literally wore out more than one copy of this book. If an activity is one your kid would not do, you know it is an activity that should not be in his sensory diet. If it is an activity he would love and beg to do over and over, look at what type of activity it is, and explore other activities of that type.
One hint that I found was a HUGE help with my sons. Make sure that they always have plenty of protein in their snacks. My brother always got mean if he had sugar without protein, so I watched my oldest with this even when he was little. I found that he got cranky. Then I found that we had meltdowns and rages if he was low on protein or he had a lot of simple carbohydrates or a lot of sugar and not a lot of protein with it. I saw his little brother was just unable to function without protein. He didn't get mean, he just didn't cope with anything. I kept protein bars that were a balance of 40% carb, 30% protein, and 30% fat or had more protein than that in them with me at all times. If we stopped for food after school, they didn't get sweets. They got something with protein. If we had to do errands or go somewhere after school, I made sure there was food with protein. If I didn't, the boys were going to have a meltdown. It was a guarantee. If I had a source of protein, they were fine.
I found it very interesting to watch them when they got old enough to choose their own foods and to start to rebel against the rules about protein snacks. I always said it was fine to make choices, but if they had a meltdown and they didn't have protein, the rule about protein would go back into effect. My oldest had to have to rule put back in place a couple of times, then he started realizing he felt better when he followed the rule. He stopped trying to change it then. My youngest broke it a time or two and came to me to talk about it one night. He told me he had worked so hard to not have a meltdown over simple requests and ordinary things like "please pass the salt" that he thought that maybe he needed the protein rule to be a life rule for himself.
You also might consider seeking out a developmental pediatrician. That is the specialist who treats autism in my neck of the woods. They are hard to find, usually located in children's or university hospitals. You also could check out some of the autism parent groups in your nearest big city. Many have a facebook presence and even monthly meetings. You could ask them who the best doctors for diagnosing and treating are, and who the best therapists are for the many types of therapists that can be recommended. Often this is a great way to find out who is good and who is not so good in your area.
Waiting lists can be very very long. Always ask for the cancellation list. Remember to call in every week or two (whenever you can) to ask again if there has been a cancellation. Always be nice to the phone people as they are the gatekeepers to the doctor. You often have to be nice to the person at the gate to be able to get to the doctor. This is a case of the squeaky wheel getting the grease. I often found that the phone person got tired of talking to me and found a spot to keep me from calling again and again. I don't know why, I was always really sweet. And on appointments near holidays, I brought cookies for the entire office! But only after they got me in for that first appointment.