I am assuming that you are feeding him enough protein, using a good quality chicken feed. A lot of his muscle development will depend on his genetics, and you may get a bit more through diet, but a leghorn rooster, for example, is not ever going to have a great deal of meat, compared to a dual purpose or meat bird.
Currently I have a bachelor pad, and they need to be processed. I am giving them a bit more fermented scratch, as I too am hoping to put weight on them, but it will be a more fat bird. I am hoping that as I have kept them in a smaller run/coop and not let them free range, they won't be as tough. But I already know, that while these crossbreds will have some meat on them, once I get the feathers off, they will look scrawny compared to a grocery store chicken.
You want more muscle, more lean meat, you need to have the right breeding.
MRs K