I don’t have much information about your son, so I would be crawling way out on a limb with advice. A few things popped out though, why oh why would the people at a day program let someone wander around wet? This is a recipe for skin issues. The second thing is that he does not do it at home, so this is more than just a schedule change issue. It isn’t like he decided that he has to do this every day in the afternoon.
Do you feel like you could have a frank conversation with the day program?