I’m unsure what that might be, but sores around/in the mouth is associated with rabbit syphilis, which is a treatable condition. As a young rabbit, it would have been something his mother was carrying and it was naturally passed on. If it is syphilis and is becoming symptomatic, you would need vet intervention to ensure the sores/blisters are treated, as they can get pretty icky and painful: http://www.medirabbit.com/EN/Skin_diseases/Bacterial/Syphilis.htm
I do wonder if rabbits would naturally get a sore like that if he accidentally bit or hit himself against something?
I would continue to monitor and if it seems to be getting worse, vet attention would be wise. It might be something innocuous which resolves on its own.
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.