Be glad he doesn't eat too much! Most large dogs are over-estimated in how much they should truly eat - as long as your vet think he's in good body condition, then that's fine (you should be able to feel
ribsRib cage pain on him, but not see them - see Purina's Body Condition Score website for helpful info).
As for the lack of muscling in his backlegs, I have seen this in large breed dogs - some times it's just the way they are, sometimes it's muscle atropy if they have bad
hipHip joint replacement
Hip pain dysplasia. Just to be on the
safeSafe driving for teens
Safe sex side, since he's a giant breed dog, would be worth having xrays done at your vet to have him assessed for
hipHip joint replacement
Hip pain dysplasia.
Otherwise, if they look good, he just has skinny legs!