Nice horse Wooki.....but I agree with BB, hes got a pot belly or grass belly. (Bit like those starving african kids, all skinny with a big belly cause there not eating enough.....lol....)
I would increase his hay, Lotsa hay...
you cant go wrong!!!
Hard feed (try something simular)
Barley, or Lupins or full fat soy, or pellets
A horse getting ridden 6 times a week needs to be fed accordingly. I believe you can exercise a horse but he wont put on muscle without the right feeding.
No matter how much long and low riding you do if your horse is burning all his energy then he will drop weight were it counts the most.
You say he keeps his weight on, but to me looks like in all the wrong places. Try adding more protein into his feed and see how you go