My boy had a impacted anal gland when he was around 6 months. They did drain it. The tech described the smell as being fishy, a odor you can't mistake for gas. Luckily we have not had any other issues.
Honestly, the gland could be infected. I would take him to the vet and have them take a look. I know some people drain them on their own.
Is his poop always soft? They should naturally drain when they poop.