C.C.
My step-daughter did that at about the same age. She would wet herself at school, sometimes even twice a day, because she just didn't feel like taking the time out from whatever fun thing she was doing to go to the bathroom. Eventually my husband lied and told her we weren't going to bring any more clean clothes to school for her to change into (we still did, but told her teachers not to tell her either). She miraculously regained control of her bladder and didn't have any more accidents.
Since your son is at home you can't exactly tell him that there isn't anything to change into but you can go out of your way to make it a real pain in the butt for him. I wouldn't go so far as to punish him at this point but certainly make him clean himself up, clean the floor/chair, put his dirty clothes in the washer, etc. Hopefully he'll realize it's less trouble just to use the potty.