What Causes Severe Night Sweats In A 21 Year Old?

hi doc. my bf has had these night sweats for about 4 or 5 years hes 21 & its so much sweat to where its on me! he wakes up so much at night. its messing up his sleep, i feel bad. we thought it would stop even if its cold in the room hes stil sweating. he says hes cold so he uses the blanket but hes sweating? i dont understand. can you help?