Sweat!

So my boyfriend sweats in his sleep. A lot. Last night I woke up and our blanket was soaked on his side. I have no idea why he sweats like this, and he says he's not hot. I'm not sure what to do about it other than give him a thinner separate blanket and put a plastic cover under the sheet so the bed doesn't get nasty. Has anyone else had this problem or have any other suggestions or solutions??