J.S.
This could certainly be any number of things. If he's not a very good teeth brusher, he could have tons of tartar and plaque buildup...deep into the gums that a toothbrush won't get out. He may just need a good deep cleaning (different than a regular 6 month cleaning) and some oral hygiene instructions. I would visit a dentist first...this would be very easy to rule out. If that's not it, then definitely see a doctor.