My dog is really thin how can i bulk him up?

Hes my moms dog he is a papillon, and he is about nine years old. He eats his dog food just fine and every now and then he gets a few table scraps. But he is very thin and you can feel his rib and hip bones. He does not seem to be in any pain he still likes to go on walks and play with his toys. I was just wondering if thats normal for his breed and age, and if there was a way to maby get him to start chunkin on the pounds?