Sometimes it may be different when it comes from grandparents vs. the parents.
He is just out of his teen years, when he was living home 10 months ago, dynamics you had with him from his earlier teen years were still at place between you and him. No parents and teens get through teen years without collecting some baggage in to their relationships. Often grandparents and the kid do not have that hindering their relationship. And also: It was ten months ago. He may have learnt little something during that time even if it doesn't seem like that.
With our Ache laziness has never been one of the main problems and he does have structured lifestyle, but still, when he has at times been at home and worked in some project with his granddad, or just gone fishing with him, it has worked better than any anti-anxiety medication could. The way he has calmed down, settled and seemed to have found some peace have been remarkable. To me it just seems like that type of life, regular, involving physical work, preferably with someone you care about a lot and at outdoors, just seems to be very healing to many. I know nothing relaxes me more than waking up at the morning, having breakfast and heading to forest to pick berries for the day. It just makes time and that everyday nagging anxiety to disappear. I myself try to use at least week of my summer vacation every year just for that.
And Jabber, I'm sure that you are right and their plan is doable, but let's face it. It involves finding at least semi-stable job, saving some money, doing lots of legwork to find a flat that would accept them and so on. If he actually successfully goes through all that, aren't we going to declare him almost an ex-difficult child then?