I'd wait for more experienced answers, but my initial feeling is that if he has a winter coat, as long has he has constant access to hay he should be all right. Horses produce more body heat when they are grazing, so just make sure that he has hay available.
The important thing (if you choose to blanket) is to make sure that he doesn't sweat underneath, since a wet horse is never a warm horse!
 Just re-read that he is going to be stalled! In that case I wouldn't worry about him. If in doubt, try the light sheet.
Last edited by existentialpony; 10-23-2013 at 12:53 AM.