Buy straight lard mate (they love it aswell), mix it in with his dinner if you really want to stack it on, if you can ask your pet food store if they have a higher fat content chicken mince as this depends on the amount of fat/skin put in with the mince. Depends on the store, and you will be able to tell by the pink/ white in the chicken, if its red it is as lean as you get chicken. I also agree with the pasta comment that will help him fill out.
Other than that I suggest that you spoil yourself and get yourself some nice steaks with fat left on and when you cook it cut off the fat and give it to him
he will love you for it. Another option is to get him a winter weatherbeeta rug, it will cost you around the 50 dollar mark and being a racer he will have no problem adorning the clothing and it will keep him much warmer.