Son may have a yeast infection he has been on amoxicillin now he has a rash on his butt and penis is red swelled up it doesn't look like diaper rash?

If he has...: ...A rash on his buttocks and genitals, and he is in diapers, it's a diaper rash by definition. Fungus and yeast can cause diaper rashes. Your pediatrician can tell the difference between a fungal diaper rash and one that is simply irritation. If it's fungal, fungus medicine will cure it. In the future, give probiotic supplements when giving antibiotics. They will prevent this problem.

Candida : If your child on oral antibiotics like Amoxicillin may develop a candida dermatitis in his diaper area. This would not be unusual. Candida is usually treated with nystatin. See your pediatrician if the rash gets worse. Good luck!

Quite possibly yeast: Yeast rash is a frequent complication of the use of antibiotics in children. No one antibiotic is necessarily worse than another--but prolonged use of antibiotics seems to more often trigger yeast rashes. Thrush or oral candidiasis often can presage yeast rashes as well. You might want to look for milky white patches on your son's inner cheek (inside his mouth); it does not wipe off. See your md.