Is he on Prosobee no iron or low iron? Maybe if it's no iron he needs low iron instead. My son had to switch to soy also because then Enfamil wasn't sitting well on his tummy and no iron was making his poop runny so we settled on low iron and gave it time which eventually worked better. Just keep your pediatrician updated so they can better advise you and keep record. I hope you find a formula that works for him.
There are other types of the formula, lactose free, etc or maybe u should try and diff brand...hospital started my first on enfamil and he kept spitting up and vomiting...I switched to similac and he stopped, and also was on the lactose free version. He had a horrible bleeding diaper rash that went away once he was switched....good luck...its trial and error unfortunately
If the rash is on his bum he could be having just very acidic poop/pee. It happened to my daughter. She always had a rash. We switched from using a diaper rash cream to coconut oil (since its a soft solid a room temperature) and haven't had the rash problem since. Going over a year of a constant rash/redness to nothing just by switxhing to coconut oil. It was amazing!! I plan on using nothing but coconut oil with this baby too.
My son alsp had real bad nappy rash. The only thing i could do with change regularly and use a zinc based cream with evry change. Also having some time each day with no nappy helps as well.