To treat your 13-month-old son's redness and bumps on his penis and testicles, it is recommended to let the area air out by reducing diaper use during the day, allowing him to go diaper-free for periods of time. This can help reduce moisture on the skin and may resolve the issue within 3 to 5 days. Avoid using zinc cream during this time. If there is no improvement after 5 to 7 days, you may consider reapplying the zinc cream and seeking advice from a doctor. It is important to avoid using cortisone cream on such delicate skin unless prescribed by a doctor.