There are a few different potential causes for this condition from the perspective of traditional Chinese medicine. The Spleen is in charge of three things. Digestion, keeping the organs held up in the body and finally, keeping the blood within the blood vessels. This third issue is the one that effects your son. If we see other signs or symptoms of Spleen Qi deficiency, then the treatment would be directed toward this end.
Heat is a pathogen that comes from outside the body.
Western medicine might call this an infection. At least that's how it begins. Long after the flu or infection has passed, the pathogen can bury itself deeper into the body. The Blood is considered a very deep level for a pathogen to get into, but it can happen. Once that has happened, we have a condition that we call Blood level heat.