If you leave him hairy and he has enough winter hair to keep him adequately warm (and it definitely sounds like it), then I wouldn t rug him. Putting a blanket on over a winter coat can actually make it harder for their hair to do its job of insulating them. And once the hair has grown in, adding a blanket won t do much to stop it. Light has more to do with hair growth than temperature (that s why show horses are often kept under lights).
But if he s so hairy that he s frequently getting too hot (like his body is still producing hair suitable for his former climate), then consider doing a partial or full body clip and then blanketing when needed. That way, he ll stay cool enough on the days that aren t so cold, but you can always add a blanket when it is cold. If he s only sweating/getting hot every now and then, I d leave him hairy and just avoid riding or overworking him on those days.
I blanket mine (also sheet, and hood ;)) from the time it drops below about 55*F. He s also kept under lights the majority of time so that he won t grow hair (the barn lights are set on a timer to come on at 6am and go off at 10pm). So the blankets are necessary to keep him warm--he d freeze to death when it gets below freezing. But my boyfriend s horses both get to grow their own winter coats and get no blankets, because there s no need in them being slick and shiny.