You may have tried it, but if not, Aquafor or vaseline. My kids are both rashy. My one son is almost 8 months and his chin and neck are often rashy and the aquafor clears it up. Maybe he's not so much allergic to the spit up as sensitive to the wetness. (That's how my baby is.) Aquafor (the tub of really greasy feeling stuff in the regular section, not the baby section--the baby section stuff has fragrance added and is a lot more expensive) or even vaseline should help with that. Also, Aquafor makes a baby soap I like but be sure all soap is rinsed with clear water after his bath. And put the aquafor on right after the bath each day to seal in the moisture, and if he has the rash, put it on several times a day where it's bad. Also, I would use laundry detergent for sensitive skin and don't use any stain remover products--I think these make my son worse. Make sure his blanket for tummy time is washed in the sensitive skin laundry stuff, as well as any clothes on you that he's up against a lot. Don't let him on the carpet without a blanket under him. These things helped my kids, but sometimes the rashiness just comes for no good reason and i'm never sure what to do about it. My kids have/had eczema, which isn't an allergy to anything specific. He may outgrow it around a year old or so like my older son did. (But then he may end up with lots of allergies like my older son. Apparently his allergist said that eczema as a baby often means a kid will develop allergies.) If the rash is eczema or sensitive skin, hopefully this advice helps.