To get DS2 to crawl, I used to HOLD him on the floor, in a crawling position, and put a favourite toy just out of reach, so he reached for it. Then I literally moved his arms and legs so that he 'crawled' towards it, with me making him do the movements. It took MONTHS AND MONTHS of this, but due to retained muscle memory (same way soldiers get so quick at taking a shot with their guns!), eventually he mastered it.
Apparently if you repeat the same action 500 times, your muscle 'retains' a memory of it, and it becomes more natural. I swear it was more like 5,000 times for DS2, but he got there in the end.
Also, I used a kids football like an except use ball for him (he was too small for a proper one).
I'd sit him on it, and hold him up as he 'wobbled'. Strengthened his core. I would also lay him face down with his tummy on it, and sort of 'roll' him backwards and forwards over it.
I did spend HOURS every damn day for over 2 years trying to get him to first crawl, then walk.
For sitting up, I had a Galt dinosaur/crocodile cushion (was never certain which it was!) that literally was long enough to go from his left leg, round his back, and right round to his right leg, that I used to 'prop' him up on, and try to get him to reach for toys.
It's all paid off in the end though - and while his physical development is still VERY delayed, he CAN run, walk and climb. He has developed kyphosis due to the Hypermobility, so I would urge any of you with younger hypermobile DC's to keep an eye on the alignment if their spine, and for any unusual curvature of the spine as they DO learn to stand and walk. Especially after growth spurts.
It took hours and hours of perseverance mind you, I had to be very committed - I was told he'd NEVER walk by the Physio & OT!
I will say that he takes a backward slide in his physical abilities whenever he has a growth spurt too, and apparently that is quite common in hypermobile DC's.