Rash help

So my little guy has had this rash in his tush for a bit. It looked more like a blister and sometimes will bleed. Doc gave prescription for cream and told me to rotate between that and desitin every diaper change. Its cleared up 90% of the way, but not fully going away.

He is primarly breast fed. He has gotten at least 1 bottle of formula a day since he was born. We are using the enfamil that they had in the hospital. Had no problem.. switched to the powder version around 1 week and thats when it started. Stopped the powder and went back to the liquid

Any suggestions?