What are some good ways to build muscle and condition a horse

I have a 4 yr old appendix quarter horse that I am trying to build muscle on (especially on his hind quarters), and get into shape. Right now I have been lunging him every day for 25-30 mins at the trot and a little canter, and I have been riding almost every day for about 45 mins. Right now he has a big belly that wont go away, he just isn't building muscle. He is a big, stocky guy naturally, but is all flab. When I ride he goes correctly, he uses his hind end to push off, and collects up pretty well (obviously not perfect because he is somewhat green still). I am about to start showing him, and am trying to sell him, he is a great horse, really nice conformation and every thing, but he just doesn't look in shape.

Are there any suppliments or anything I could add to his diet to work kind of like a protien shake? (I don't mean steroids or anything)

any advice to more effectivly condition my horse?

What are you conditioning him for... I barrel race and this is what I do!...

Swimming is excellent conditioning. Often most people don't have that available to them. I walk my horses in the canal behind my house. It's about knee deep. The water is good for their legs. And it conditions thier muscles.

I've never tried supplements... don't want to!

I have 1000's of acres of mountains and hills behind my home. I ride out there.

Often deep sand is good (Don't lope or trot(maybe a lil) them in it... it really can be too strenuous.)