can't you buy some used wintergear for a few dollars at goodwill store ?
you can donate it again when no longer needed .
heavy socks , long johns/sweat pants (to wear under regular pants) or lined pants , windproof jacket and warm hat should see you through - make sure it's a bit oversized - if it's tight fitting it won't keep him warm .
any indoor recreation centre nearby ? even walking in the shopping mall for an hour is good exercise .
have you asked mo would he would like to do ?
stay warm !