Juno so many things can cause rashes so it is best that your lo is seen by a paed who could refer you to a dermatologist. They may run allergy tests which is possibly the most common cause of rashes. Heat can cause a rash but not near the mouth I don't think. He may have a combination of rashes caused by different things.
If he has a fever then it may be a childhood illness.
If he uses a dummy it may be from that even if his saliva is acidic.
Rather see a doc to try to find the cause.