Have you ever checked out his liver? Sometimes liver shunt babies have trouble putting on weight. I hate to bring this up, but I think I'd want him tested before you add more protein to his diet. I'd definately do a blood panel before you neuter him.
There's a recipe called Satin Balls that has been posted here many times. Breeders sometimes use a receipe that's basically hamburger and a few other things to put weight on their studs who are around females in season. The boys are so nervous it's hard for them to eat.
Satin Balls Half Recipe -- recommended amount to mix up
5 lbs cheap hamburger (for high fat %)
1/2 large box Total cereal (about 6 cups cereal)
1/2 large box uncooked oatmeal (about 7.5 cups oats)
5 raw eggs
1/2 of 15oz jar wheat germ (about 2 cups)
5 packages Knox unflavored gelatin
5/8 cup vegetable oil (this is pretty close to 2/3 cup)
5/8 cup unsulphered molasses
pinch of salt
I personally wouldn't use the molasses, it horrible for teeth, I don't think you need the oil either. You may create a monster and they won't want their regular food. I took the recipe from this post.
http://www.yorkietalk.com/forums/gen...tin-balls.html There are many variations of it on the web.