Does it bleeds regularly during brushing his teeth?
If it so first of all you need to change your brush to ultra soft kids brush (kindly change after every 3 months),
secondly, you need to see it by yourself that any depostion or collection is present on the entire dentition.
If the latter is present as a soft matter (plaque
) then kindly clean it with the brush if hard matter is there then see a paediatric dentist