Raw eggs are fine -- it's the egg white when fed alone that depletes biotin -- the yolk makes up for it though. So as long as they are fed together, it's alright.

If you go to your local petstore, you may be able to find Nutri-Cal. It's a high-calorie vitamin supplement. I give some to my dog when she starts looking a little thin because she can lose weight so dramatically. It can put on weight quickly. You should also look up a recipe for Satin Balls. Great for fattening dogs up. Just don't feed too many of them or any other treat -- take it slowly.