He probably just has a little virus or small infection in his body. Their young immune systems react to everything and I wouldn't worry too much. My oldest son, who is now 42, had huge lymph nodes in his neck and he was pale and I worried so. His was mostly due to his tonsils. We always imagine the worst and thank goodness, it's usually not the case. If the Dr. wants to do blood tests, have them done to ease your mind. I'm sure everything will be ok. Keep us posted