I'm going to guess that he has some mental health issues that haven't been dealt with. No one wants to actually live the way that he does. He sounds depressed to me, and possibly addicted to gaming (or who only knows what else). Did he get help in school? Was he evaluated for things like learning disabilities or attention problems? What did those tests reveal, if anything?
Normally I'm a proponent of tough love but he sounds like he needs help. Get him into see a doctor ASAP to assess his physical and mental health and get him therapy or medication if needed until he's got some control over his life. If he refuses to go to basic training (and it doesn't sound like he'll succeed in that and will wash out in the first few weeks or months anyway) AND refuses other help, then you can take away his house keys, lock up, and if he shows up trying to get in, you can refuse and call the police. That would be a pretty harsh first step though. If this isn't the first step and he's already done therapy, medication etc. and is truly just lazy and unmotivated, then go ahead and try tough love and see where he lands. Some people do land on their feet, wake up and start living and others, frankly, don't.
He says he's got mental issues...what are you doing about those? What have you already done? He doesn't sound like a lazy ingrate to me, he sounds like someone who desperately needs help and has needed help for years. I would start with that before throwing him out.