What discipline do you do with him (sorry new here so have no idea!!)
I can help you from an english perspective though so bad luck if you're western :P
I'd be working him quite deep and have to really travelling forward to stretch out those back muscles (topline). Low and long work would benifit him, but not the 'beginners' version of long and low- aka put horses head as low as possible to the ground but don't worry about the back end. That will do nothing but build up more under muscle and you'll lose muscle behind too as he'll be relying on his neck to balance him being on the forehand.
Lots of leg, hold your reins out nice and low and wide, keep a nice soft contact and push him into it, holding him with your back so he doesn't run off on you. Lots of transitions between paces and within paces to really start working those back muscles and getting them to stretch and build up.
If you're not able to ride at the moment, does he know how to lunge? How experienced are you at lunging? If you've got some experience behind you and he knows how to work into a contact without falling on his head, pop some lose side reins on him (no body hurt me for saying the dreaded 's' word, I have yet to have a horse with a stuffed mouth from the, when used correctly) and lunge him lightly. AGain, plently of transitions, changes of direction etc. and make sure he's got correct flexion, staying nice and round.
Good work with some decent feed and he's topline will build up in no time ;)