If it's on the shaft, you can clean the area and use a hypoallergenic, non-scented lotion (Curel). If it is on the head and he is circumcised, he can do the same, but do not get it inside the penis. If he is not circumcised and it is on the head, he can rinse the area with water once a day but don't add any lotion of anything. The foreskin's purpose it so protect the head of the penis, sanitize it, and keep it moisturized, so it should heal itself.
If you are looking for something a little more natural, you can use olive oil mixed with a tiny bit of the juice from some crushed garlic. (Garlic is a natural antibacterial agent and olive oil will moisturize the area and protect it.)
If it hasn't healed up by Monday, I'd call the doctor. A rash in that area can become infected even if cared for properly.