My 4 month son has swollen lymph nodes on one side of his neck. They are small, movable, and shrinking slowly. Should I be concerned? He had a cold

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.
Cold virus. can enlarge lymphnodes in the neck area. It would gradually will go away.