My 2 year old son has what appears to be a cold sore below the left corner of his lip. It has went away and come back a couple times.?

May be herpetic. Without seeing this it is difficult to diagnose, but herpes simplex is a common virus which causes cold sores, and it may recur periodically for the remainder of your son's life. This virus is like a squatter. It sits in your body, sometimes silently without producing any symptoms, but may recur with stress, other infections, or sometimes without an obvious cause.