Hi,
My son is nearly 4, now, and has hypotonia and joint hypermobility. At 15 months, he could only sit if put in that position or lie on his back. He was always clearly bright, had good fine motor skills and his receptive language skills were very advanced, but he couldn't really talk clearly until he was 3. He can now walk, run and jump, talks eloquently, sings in tune, and can already read fluently, with understanding and expression. He can also do some of his times tables and do simple addition and subtraction, so cognitively, he is exceptionally advanced. His memory and concentration are also excellent.
As for what he still finds difficult... He cannot really hop, yet, or skip, and whilst he can pedal a tricycle indoors, he can't get the pedals moving outdoors. He also walks quite slowly and runs slowly, but has the right technique. If he uses the correct pencil grip, he can only write and draw with a fat felt tip pen - he can't make much of a mark with a pencil unless he adopts a slightly more unusual grip. However, he can form all his letters and numbers - provided you're not too fussy about the size or positioning on the page, but then he's only 3, so that's pretty good going. He is not keen on drawing, or playdough, or other activities that might strengthen his hands up. He is also left handed, so I predict he will find learning to write quite tiring and frustrating, as well as using a knife and fork properly, as cutting up food requires quite a good grip and reasonable hand strength. However, he does build up strength gradually, so it shouldn't be a lifelong disability, just something he'll have to work hard at.
Even at 4, it isn't clear exactly what is causing his problems, but more serious conditions have ruled themselves out - all that's left, really, are mild connective tissue disorders, unbelievably rare myopathies (the least likely possibility), or something like aspergers, which can be associated with hypotonia. He has some unusual personality traits and social skills, and does seem to need to be taught a lot of his motor skills, rather than having them come naturally to him, but really, he isn't clearly anything at age 4, besides hypotonic and hypermobile, so we will just have to wait and see a bit longer. A lot of the more autistic-like behaviours he has already grown out of, or seems able to keep under control, so he would be on the very mild end of the spectrum if he is, and part of the needing to be taught his motor skills may be a personality and confidence issue, as there aren't any age-appropriate skills he hasn't been able to master through intensive teaching/bossing-by-mummy, yet, except for the hopping and skipping.
All a bit of a mystery, really! I know he'll never be an athlete, though.