My son has a lump on the right side of his neck. What should I do?

Lymph node. There are thousands of lymph node located around the body that help screen out germs in the body serum before it returns to the circulation. There are many in the neck, head, behind the ear. At rest these are often 1/4 inch in diameter and may grow to marble sized when reacting to a stimulus. Bug bites, or viral infections can increase their size.They should shrink down to nl size over a few wks.
Watch it. Lumps on the neck are usually lymph nodes. These are factories that collect debris along the lymphatic tract. When we are sick, the nodes increase in size and work to clean things up. If your son has had a cold, sore throat, scalp sores, the node may be reacting to this. If he is not ill, the lump is red warm and hard, or the lump is not softening in the next few weeks get him checked.