How can I help make my pug mix gain some weight?

How old is your pug? Why is your pug underweight and by how much? Have you seen your vet? What measures have you already taken?

Yes, a little more info would be helpful. I'm curious to know why he is underweight, and the circumstances?

It could just be worms but it could be something more serious too. When was your dog last wormed? More info please