He may be just homesick, but like sewmuchmore suggested, talk to the school and make sure everything's going okay, more important though I think, is find out if he's make any friends and talk to them. My granddaughter's school had a couple of exchange students, one of them happened to be a very soft hearted Korean boy, and my granddaughter said he was be harassed by some boys, so bad that he would actually start crying. She told the principal and the boys were "talked" to, but I think that he was probably glad to go home. I'm sure you'll get some good suggestions though.