2 yr old's life jacket strap blistered inner leg...suggestions on healing it?

My husband allowed our son to swim the other day with just the life jacket on and swim diaper, instead of his swim trunks. He said they were in the pool for about 2 hours. When he got him out he realized the life jacket strap that goes between his legs had rubbed on him and he has about a 2 inch in length blister. Its in the inner part of his legs...really really close to his private. I'm putting ointment on it and trying to keep it clean (not letting him go long between changes and quickly change after poop) What else can I do? What type of cream should I use? It hurts him only when cleaning it and putting medicine on. How long do these things take to get rid of? I am not familiar with blisters at all! Any input would be wonderful :)