Don't label it for husband. Just say, "I think he has some sort of problem remembering things and staying on task. Or do you think there could be another explanation? Because he should have been able to write down that answer and move on. What do you suggest, to help him get his work done more effectively?"
Think about it tis way - if all the kids difficult child's age are like this when doing maths in class, how long is each maths lesson going to be? How effective is the learning going to be? Clearly, difficult child was not, at the time you describe, working effectively. He was clearly putting in a lot of effort, but it wasn't showing up in results. And difficult child was getting frustrated - with himself, as much as anything else. He had the answer and didn't trust it, he was needing at some level to verify it in his own head, but couldn't hold the various bits of information in his head and manipulate them, while still remembering what he was trying to do.
Yes, it's an inattentiveness problem but it goes deeper than that - to memory and how it's working. So if you talk to husband about your concerns for difficult child's ability to manage bits of recall, manipulate them and produce an answer - point out that this is only going to become a bigger problem as schoolwork gets more complex. difficult child could have prodigious maths ability, but with this memory problem, be unable to use it or demonstrate it.
Don't mention ADD. OK, easy child 2/difficult child 2 had almost identical problems to what you describe. difficult child 1 had similar issues but in a quiet room could at least do basic mats. He needed remedial coaching at one point, to help him catch up once we got his diagnosis and treatment in place. For both of them, this particular problem was put at the door of ADD. But the observation - a memory dysfunction, something wrong with how the memory paths are laid down in the first place. It needs careful assessment but can be almost magically treated - different learning techniques, if you want to avoid the topic of medication (although for us, medications fixes the memory problems like magic).
You need him as on-side as he can get, so only tell him what form of the truth he can handle, and that will encourage him to want to get difficult child some help also.
If he had vision problems, surely husband would want him to get his eyes checked, maybe put up with glasses for schoolwork, at least? This is really no different.
I get really concerned when community attitudes towards things like autism, ADHD etc view these as disability, brain damage, dysfunction. These concepts are very negative and can really get in the way of help being sought (or made available), the child's personal expectations and self-esteem. When parents see these conditions as a disastrous diagnosis, they often reject the whole idea and the child goes longer without the adaptations being made that will help them best utilise the different way their brains work.
I'm a left-hander. I copied my older sister, I probably shouldn't have been a left-hander, but because I was reading/writing well before I started school, I was well into the habit of using my left hand. My older sister was in the generation where you MUST use your right hand, and went through purgatory because the system discriminated against her. She developed a bad stammer (which we now know sometimes happens when brain dominance is made to switch from one side to another) which took a year or more to resolve.
I was much younger and in my era I was permitted to use my left hand to write. Teachers weren't happy about it and kept making a show of how untidy my work was, but I was happier using my left hand and was prepared to accept the disadvantages of critical teachers, as long as they let me do things my own way. My sewing teacher showed me to run the stitching in the other direction, so I became quite skilled despite being left-handed. A later teacher discriminated against me badly in class - left-handed the reason? I don't know.
I do not consider being left-handed to be a disadvantage or a disability, but in years past, it was seen as such and treated as such.
When we 'see' ADHD as simply a different brain wiring, or a different way of learning, we get more results in terms of cooperation from teachers, family, others. The child feels better about themselves once they realise it's not their fault. It also moves us away from the expectation that the child will be otherwise completely normal, when this is not the case - being different in ANY way colours every aspect of your life and the more you are forced to be different to your own nature, the more you feel a fraud, a fake and a failure.
Somewhere in your son is a highly intelligent kid feeling very frustrated, angry with himself and angry that he is expected to do stuff he just can't stay on top of. There are better ways for him and once they are found, he will do better and feel happier. husband needs to know his son is bright, capable, inventive and a good son. He is afraid that a label, ANY label, will take away the golden image he has of his boy. It's a bloke thing. Your husband is also probably afraid that a diagnosis for your son means eyes and fingers on husband - loss of face? Loss of manhood? Again, it's a bloke thing. "If my son is imperfect, what sort of man does that make me?"
Yes, it often comes down to what the man thinks of himself and what the implications of everything else are, in terms of how he feels about himself. Apologies to all the wonderful men who post on this site, but I think you know men like this, when I say - it's a bloke thing.
Kathrine, you don't need confirmation from us. You need support from husband, but frankly, I think he's too scared to face it. If you present it as "he's wonderful, but may need some support to find how HE works best with that amazing brain of his," you might get a shade more acceptance. Or you can simply stop trying to get husband to agree with you - as long as he agrees to cover the medical bills for the assessments etc it could be the best you can hope for, for now. Then once you get a diagnosis, if husband disagrees tell him to argue with the experts, not you. husband can go see the person who diagnoses your son and discuss it with them. Maybe they can make husband see sense. And if he refuses to go - then he's got no right to disagree. When did HE get his degree?
Hang in there. How long now until the appointment? Have you talked with difficult child about any of this? Or are you waiting to see how it goes?
Marg