Supplements to build muscle - thoughts?

I recently accquired a 12 year old american warmblood who needs some serious muscling, about 100 pounds worth. He's just very underconditioned in general, and he needs all the help he can get :) I have had great success with weight building supplements, and he'll be on one for a while, but in the long run he's going to need muscle more than fat.

What are your thoughts on muscle supplements? Do they work, I realize that nothing replaces consistent collected exercise for building muscle, but all our fields/riding ring get pretty icy this time of year, and I want something to boost the muscle process.

I have heard good things about Fat-Cat from Vapco in the past - any success or horror stories you have with this product, please share them! If you use a different supplement for muscle development, tell me!

Thanks in advance! As soon as I can find the dang camera cord, I'll post some pictures, b/c we all know that makes these posts more interesting :)

I don't know much about supplements and all that but I would imagine anything with a healthy amount of protein will help a lot. Put him on that and do a little bit of light work, maybe some easy lunging, and that should help build up his muscles quite a bit.