I don't have a pug, but I recently fostered and trained one. She was very smart and so cute. She was rather easy to house train, we kept her on a strict schedule. I taught her "go pee pee" and she would go find a place to pee and poo. We gave her a treat after she went potty. We also crated her at night and while we were out of the house.
I only had her a few weeks and in that time she learned "go pee pee", "calm", "here", "sit", "stay".

Just work with him and keep him on a schedule, he will learn.