Cold sores are contracted by saliva. He could have picked it up from any one even you. The bacteria is still in your system. He could have picked it up easily from other kids.
This is what I found on catching it.
The herpes simplex virus is contagious and can be spread by saliva. The most contagious time is when the blisters are fresh – kissing others or sharing toothbrushes, drinking glasses or cutlery should be avoided during this period. In particular, the infected person should be kept away from newborn babies, children with burns or eczema, and people with suppressed immune systems. However, it is possible for an infected person to pass the virus onto someone else without presenting any obvious symptoms. This is because the virus may be lying dormant in the skin cells of the lips. A person newly infected with the herpes simplex virus typically develops a mouth infection. The virus then inhabits the nervous system, and may or may not produce cold sores, depending on the individual’s immune system. A person prone to cold sores should avoid kissing newborn babies.
I am not sure of treatment for someone his age but he does need something. If you think it is a cold sore take him to the doctor. Good Luck
Thank you for the information. When I got home last night he didn't have anything on his lip. Maybe it was a hive from the dog or maybe he bumped his lip. But thanks.