Dog won't gain weight

Thanks everyone for ideas I'll have to give some of these a try. I read up on the coconut oil. They say it's really good for dogs.

Also I'm looking for an new vet cause one they charge me a way to much and they give me answer like well maybe it this but dunno. Not answer I want to hear. I want to hear "you should try this or let's try this" not dunno you can. Also they told me I should first try getting him fix to and that threw me over after that.

I'm gonna try some of these ideas and go from there.