Help rash!

What can I do about this rash. We go to the doctor in a week for his four month check up so I hope to have it better and not have to go in before then. I keep his neck clean. He never has goop caked under there like you hear some babies get. He does drool a lot so I wipe it a lot to keep it clean. I also change his bib constantly. He goes through so many bibs he even wears his sisters pink ones.

Can it be yeast? Or heat rash? Or just a rash from moisture?

What should I put on it? Any advise I will take it.