This exerpt from an article from the rabbit network has always been helpful to me. Though older bunnies do just have that less lean, plump look. And younger bunnies are so active and naturally more lean. (unless overfed)
“How do you know if your bunny is fat? Many house rabbits are, and they are not at all embarrassed by it! Rabbits store fat inside their abdomen, which makes just eyeballing them for love handles ineffective. Run your hands along your rabbit’s side, feeling the rib cage. When you reach the end of the ribs, your should be able to feel a slight inward slope, the waist. Fat bunnies will balloon out instead. Remember that the ribs are palpable in even quite obese rabbits, so don’t let your bunny fool you into more snacks!”
SOURCE: http://www.rabbitnetwork.org/articles/obese.shtml
However, if you think that he is too skinny and not gaining weight properly, you could take him to the vet. Does he have any issues with poo? Parasites can sometimes cause issues with weight gain too.