This is my son's biggest issue, and either we've missed the boat despite 4 years of OT through the school or the condition simply is what it is. Typing becomes a great solution but their hands have to grow into it. My son is just now getting into typing at age 11.
What we believe is the root cause is loose joints, or what some people call hypo-mobility. Because the joints are loose, a few things happen. My son cannot get the right pressure on a pencil. And the sensory input from his fingers is lessened, because the nerve signals have to pass through the joints on the way to the brain, and not all the signals get through when the joints are loose. I was actually first alerted to this by an adult at Wrong Planet who suffers from it, and I am now sure this is the problem.
My son has always been unusually flexible, and that is probably the first clue. But not all people with the condition are, because a condition that results in rigidity can accompany this and hide the looseness.
The reality is that using the fingers is painful and stressful for many of our kids, and all the exercises in the world aren't going to change that. Not to say you shouldn't give him fun exercises to do, for muscle strength will help him, but to keep your goals realistic. My son had difficulty learning to use the hand break on his bike, still hates to tie shoes (we buy velcro), loves music but gets frustrated because he can't play at the speed dictated (they learned to play recorders at school), won't play video games because that is also affected, and so on.
Hopefully your son's condition is not as extreme as my son's is, for we are finding this a very difficult one to overcome. It holds him back in so many ways, and is the main topic of conversation at IEP's. The elementary school let him dictate or voice record assignments, but I am realizing there has been a downside to that, now that he is in middle school, in that he has never developed punctuation or capitalization habits that bear any resemblance to the correct way to do things, and the middle school is asking a lot more from him. At some point I want him to be able to communicate in writing without relying on a second person, and since he is really excited about his AlphaSmart portable typing computer, I am hoping we can clear out some of the bad habits in the near future. But this is a constant, uphill battle, and a fine line to walk. Keep him learning, keep him moving forward, while helping him work around what we now see is a real disability.
If I could go backwards, I think it would have been good to realize that this was a joint problem and to involve the pediatrician right from the start. I don't think anyone ever thought it was that serious; just an area he needed extra help with, and time to grow into. But, perhaps, being more pro-active could have done more; I have no way to know. I would discuss the potential issue of loose joints specifically with your child's pediatrician or an OT, and get a handle NOW on (a) if this is part of your son's problem and (b) if there is anything that can be proactively done about it before your son is left falling back on accomodation tools. Accomodation tools are great, don't get me wrong, but if the need can be avoided, that would be even better.
Good luck to you and your child! It really is amazing how much there is still left for the general medical community to understand about our kids.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).