I've had great results with fat cat (Equi-Supps - Home
for more info) and it's done wonders for both older horses as well as younger horses. Hill work and trotting work will compliment the amino acids in the fat cat to build true muscle and not just add filler or fat weight - neither of which is your goal. Trot work is far more important than canter work, and hill work is good for horses that will use their hind end but doing it too soon can just overmuscle the fore.
Make sure your horse is on a good quality feed - usually 10 - 11% protein for an adult (and if he's a stud he may need more though since he's not a purebred i'd assume - hope - you are gelding him?). And the that with a good hay will do the trick. Too much alfalfa can make a horse hot so be careful there.
Trot trot trot and more trot! And keep trotting! Good luck!