If you want to build muscle, trot him up hills. The less you do with him, the less muscle he will build. Don't be afraid of him stumbling, if anything, that's why he wants to go faster because you are afraid of falling which causes you to tense up.
You won't get muscle from walking. You get it from trotting. Trotting is when the horse uses the most muscles.
Also, like others said, trot up hill. It forces the horse to use its neck and back muscles, instead of false collection from draw reins or side reins. It builds A LOT of muscle. That's what I did with my horse and I saw improvement within that week. Posted via Mobile Device