Hello and thank you for the question! If your son doesnt recall to have injured them, then its neccessary to go and make some exmainations, because the swollen finger may be caused by insect puncture? ore more likely itis a sign of inflamatory disease. The first thing to do is to check the area if there is a presence of any spot/puncture. If so you should seek for emeregency care at the hospital. Otherwise you should consult a Rheumatologyst and do some more specific tests for the right diagnosis : Coplete blood cells, erythrocyte sedimentation rate, fibrinogen, and Rheumatoid Factor levels. If the results come back positive, it means that the symptomes are caused by the inflamatory condition, and it needs to be treated with specific medications ( corticosteroids, NSAID etc ). Best regards!