You mean you want him to have more muscle tone/mass, to make him grow?

There really is no way to do that. Increasing his food uptake would only lead to obesity.

If he is healthy and isn't underweight, and if he is no longer a puppy, then unfortunately there really is no way to make him "bigger" unless you make him obese.

MAYBE exercising him will give him more muscle mass, but other than that... I really don't think there's much else you can do if he is no longer growing.

Even among purebred, papered dogs with champion bloodlines you will have some dogs from the same litters smaller and bigger; its just the wonderful luck o' genetics