When you ride him do you make sure his is working properly and rounding his back and "sitting" down on his bum? My arab looked a lot like this when I got him (except thinner too), and now his back and butt are getting nicely rounded. It isn't how much you work them, but how correctly you work them.
If you don't know how to do this, then I would second anebel's comment about getting a good dressage instructor to help you. Also keep in mind that working correctly takes more energy and muscle than just going along, so expect shorter rides and build up to it. Trying too much too soon can just make them sore and create resistance down the road. Good luck! I hope this helps.