My 6 yr old son has a blister on the tip of his penis after swimming. how do i treat this?

Cause and effect...: Is you son circumcised? Does he complaint of pain when he pees? Has he had this before? It is hard to be specific about what this is without more info. I doubt it is related to swimming. In the short term you can use and antibiotic ointment/cream 3 or 4 times per day. If this doesn't help you should see a pediatrician or gp.