I have dealt with this all my life. My whole family has hyper-mobility, and now my kids' pediatrician says they have Ehlers-Danlos Syndrome, which includes the hyper-mobility component also.
Honestly, I use those really big pens that come with the huge grips, I think there is one called something like Dr. Pen or something in the pen aisle. This was a life saver for me in college. But, typing is easier by far.
I have seen the Ortho surgeon, and he said that unless it was causing a lot of problems and pain, then he wouldn't do any surgery because it can have the opposite effect and cause more issues. The best thing I have done for it it OT. They gave me different exercises, ultrasound, and some other stuff. If you don't have to go to the pediatrician, then I would call and get an eval with an OT. You could also do one with a PT, or you might find a place like where I take my son that has both in one office and they could do the eval together maybe.
I do feel for you little guy, but maybe a hand brace could help? Go to the professionals and get the help he needs since he will have this issue for his whole life. They do tend to tighten up a little bit in the late 30s...