Has he ever been outside? Our Vet explained that roundworms can live in outside the stomach and intestines, but then migrate there. So even though you treat the worms, they are still in the body. Sort of like a herpes virus that can lie dormant for years.

We use Revolution on our boys. It controls roundworms, fleas, earmites. Our boys do go out, and we were using Frontline, but the Revolution seems to be doing the job.