Not a stupid question at all but I don't think it's possible to make him "bigger". The size of a dog is predetermined by genes and such unless he was malnourished before you got him in which case proper care and good food could help him gain the maximum weight he should be.
Smaller doesn't mean less healthy. Maybe he was the runt of the litter? If your vet has recommended putting weight on him then there are lots of ways to do that. Before trying anything I think a consult with your vet might be in order.....your dog might be small in stature but huge in heart....