An enlarged spleen may be indicative of mono and your son is definitely the age in which this hits. Is he tired and has he lost his appetite???The passing out is from vasovagal syncope in which there is a sudden drop in blood pressure due to some type of external stimuli...fever, pain, illness coming on, heat, bright lighting etc.
I would definitely ask about the possibility of mono....it's a viral condition that just runs it's course but takes sometimes up to 8 weeks to resolve itself.
Good luck and with an enlarged spleen you need to limit all physical activity since it may rupture....so be careful.....Goody