We have the same with ds1. He has general hypotonia, but is particularly weak in his trunk and upper body - he has really poor core strength. He's much the same at school, slumping, curling up on his seat, lying instead of sitting if he gets the change. His preferred position for writing or eating etc (basically anytime he has to sit up at a table) is to support himself by using one hand on the back/side of the seat to hold him up.
I felt so guilty when we found out, as we had absolutely no idea and were constantly telling him off for slouching, slumping and laying around the place. 
In order to improve his core strength we were advised to get him a wobble cushion (wedge didn't work for him as it just kept tipping him forwards) and a wobble/balance board to use at home. We also had some private pilates lessons for him, which were fantastic and definitely helped, but were expensive and we've met a lot of resistance getting him to practise the exercises at home. He did it for a month or so, but then flatly refused.
The OT that assessed ds said any exercise that requires balance is automatically working on core strength and suggested a martial art might be good for ds. Dh is supposed to be going to family Tae Kwon Do with him, but has so far failed to organise it. I'd like to try him with Tai Chi, but can't find any classes for children locally. Personally, I think horse riding would help, I know from my own experience that it really strengthens the core.
Its also important that they can put their feet on the floor or have a footrest, so that they can position themselves correctly.
I have just discovered when he was doing his homework this evening, that if I put two cushions behind him, he sits much better at the dining table and doesn't feel the need to hold himself up, so we're going to see how that goes for a while.
I'd love to buy ds one of these ball chairs to use at home, as not only would it help him to sit to the table, it would work on his core strength at the same time. Unfortunately, there's no room for it in our house.