my parents had one simple rule when almost all of my life. i had to have an afterschool activity year round. it didn't matter what it was, if it changed season to season. i just had to have a regular activity. there could be more than one but at least one. didn't matter if it was playing an instrument, swimming, chess, school sport, art class, etc.
some of those may not sound very exciting to you but they'll get him out of the house. they're not all going to get him fit and athletic but maybe he's more of a creative sort. any organized activity will provide structure and new friends.
this way, you've set some boundaries but you're giving him a choice. you're also opening the door to trying more new things.