His fur colour looks perfectly german shepherd-like to me. And his face structure is typical of that of a german shepherd mix and that of a chow mix. His eyes look like a chow's. I can't see a mastiff and an elkhound being a feesible mix. You'd have to have both dogs intact and have them somehow get together. You really need to look at breeds that are typical strays in your area. You aren't even sure that the supposed mastiff/elkhoud in the pictures is actually that breed. I'll bet that pup came from a shelter also and was just a guess.