I would just let him stay with you for a little while and maybe just encourage the military thing if he is very passionate about going back. Here is my reasoning:
1. I am 21 years old and I still consider myself as a child. I live with my parents, I listen to what they tell me to do, and I am clearly not ready to be on my own yet. If he is only 18, he is in the same boat that I am in there. I still consider him as a kid that needs to rely on other people.
2. He obviously has no job nor finances. If he doesn't have you, he doesn't have anyone. He'll be out on the streets or hopping from friend's house to friend's house and that is no way to live. That is very unstable. I know you shouldn't be responsible for that, but it's the way it is.
3. His mother and stepfather sound extremely strict, to the point of almost mistreating him. I don't have children, but I know for a fact that I would never tell them that they are only allowed in certain portion of the home. That is just crazy. And seriously, paying for his own groceries? That just seems wrong to me unless the kid would have been in his upper twenties and is making his own living, paying rent, and can pay them for the groceries. He obviously has no income, so how is he supposed to magically get this money? That is almost encouraging him to steal or get himself into trouble by them asking for him to pay for things with money he doesn't have.
4. You obviously had a huge impact on this boy. If I had a problem with someone's parent, I honestly don't think I would even remember to go apologize to them years later. Obviously your attitude towards him really made him learn the true meaning of respect and I truly think it sounds like he matured a lot because of you.
I am young myself and I know this may be of no value to you, but I really feel bad for this boy. When I was in high school, many of the people that I knew had the same sort of relationships with their parents and stayed with friends on and off. Now while I am in college, these kids are doing nothing with their lives or got themselves in a lot of trouble. I know it isn't your responsibility, but it sounds like he needs a good role model and that he really took to you. As long as he isn't causing any problems in your home, I would just say to let him stay with you. Instead of taking the route of impossibly charging him money, just ask him to help around the house and contribute like I am sure your kids do. Then hopefully in a few months he will be feeling better and going back into the military. Then he can be independent and won't have to rely on you any longer. If he decides not to go back into the military, then you might have to nudge him to get a job and go on his own.
Hope this helps, it's only my opinion.
