Your son's penis will be quite red for a few days. This should disappear within about a week. If it gets worse, if you see any crusted sores with cloudy fluid, or if you're concerned, call your doctor.
Keep your son’s penis clean, especially after a dirty diaper. Use just water, or a mild cleanser and water, as needed. Dab a little petroleum jelly on the tip to keep it from sticking to the diaper.