Don't worry. It may be mild eczema--try nonprescription (1%) hydrocortisone twice a day for a week. If it helps, ask your doctor whether to continue. It may also be a hereditary bumpy condition called keratosis pilaris. Do you or his father have bumps on the outside of your arms? In kids no treatment is worthwhile, since it's just the way the skin looks, and will likely go away over time.
Best.
Dr. Rockoff