Nutrena Empower Boost.. worth it?

I just bought a new horse. I don't know much about him other than he is approximately 7-10 years old, Quarter Horse Cross, gelding, and he's relatively small in stature (about 15hh). He was very much underweight when I first brought him home about 2 months ago. I switched him over to Nutrena SafeChoice and gave him lots of hay. He's put on a good 75 pounds since then. He's gained a healthy level of body fat, but I need to help him put on some muscle without overworking him and burning off all weight I've just put on him. I've been riding him moderately 3-4 times a week for about an hour at a time. Mostly walking and trotting. I've also put him in the roundpen a little (though, he's not too good at lounging but we're working on it)... So I was thinking about adding Nutrena Empower Boost to his daily feed? What do you guys think about it? Would this be good for muscle mass? Thanks...

I have had very good luck with it. It is a great source of fat.