Diaper rash

Sorry this is really not pregnancy related but my baby boy has an everlasting diaper rash that I can't get rid of! I've tried Burts bees diaper cream it's not helping us just started trying coconut oil so far it looks the same irritated. I'm considering melting my frozen breastmilk and using that on his little bum. His rash is in the crack so it looks so sore sorry if tmi😞 I feel bad it must hurt him. Any ideas? Should I just buy desitin will it work?