My 7 year old has pimple or blisters on his gums. What should I do?

Fistula/infection. You child has an infection associated with one of his teeth that is draining through a fistula on the gums. If the fistula is associated with a primary tooth, the infection could be cleared up through a simple extraction, however, several things need to be considered, such as the health of the permanent tooth following and maintaining space for the permanent tooth once the primary tooth is missing.
Definitey blister. -a canker sore is a painful, open sore in the mouth. Canker sores are white or yellow and surrounded by a bright red area. They are not cancerous. -oral herpes is an infection of the lips, mouth, or gums due to the herpes simplex virus. It causes small, painful blisters commonly called cold sores or fever blisters. - could be herpes, abscess, etc. Consult md.
Seek care. Your child needs an evaluation by a dentist to determine the cause-tooth or gum or both.
Could be abscess. Has he had his teeth examined recently? Could be a cavity which has caused the baby tooth in that area to become non-vital. Take him to see his dentist to have it examined and treated.