Oh, this is a problem for several reasons. The main one being that he is now 18 and you really can't require him to seek medical treatment (which he might need).
And while your partner does not like this (kind of understandable), he really has no say in the matter. This is your son.
I would look into volunteer work that you son might be able to get into. Especially those with interaction with nature or animals and not necessarily with people. For example, at the Conservancy where I volunteer we have trail crews. They go out and repair trails. Very little social interaction, but are doing a great good. Plus, its the kind of thing that looks really good on any job or college application. And most volunteer jobs let you pick your work hours and days - so its very flexible. It might take him awhile to find the right place to volunteer. Let him know that its ok to say he doesn't like it and try another once he has given it a chance. And it is certainly ok, to start out just doing a bit at a time.
And finally, don't give him a choice. He is living rent and food free. Either he gets a job or he volunteers. It is very, very bad for him to spend all his time in the bedroom on his xbox.