nasty rash! HELP

ok this is the third time my son has woken up with a huge rash on his butt just from peeing overnight. couple weeks ago he woke up from a dirty diaper and had a rash and it started to spread but went away after a week. Then this morning he woke up crying and by the way he acted his butt was hurting so I changed his diaper (it was only wet) and this same rash is back and its bigger and but this time it had a blister in it. He didn't have a rash yesterday. it comes over night when he wets his diapers. what could it be from? hes not potty trained so its not like I can just tell him not to pee his pants. and he only breaks out on his butt below his junk.

About Aundreya
Birth: July 24
On since: Jan 9, 2014
I got pregnant at 18, had him month after I turned 19. Currently 21. I work full time, full time student and full time momma to Mr. Carter Layne Kennedy. He was born august 23rd,2012. Was born 2w3d early. 6lbs 7oz 21in. I had a VERY hard pregnancy.