Lexie, I am very sorry about your situation. I don't really have any good experience to fall back on here. My daughters are both adults in good married situations. Their problems are with their mother.
As for the military. It can be a good place to learn self discipline. I served in the Marine Corps, which by the way I would not recommend for you son. The military has a lot of good opportunities for young people in an environment that is structured but still allows some freedom for growth.
What you have to be aware of is that they expect their members to step up and take responsibility for themselves. There is guidance and support from Sr people but they will not take excuses and there will be consequences for your actions. From what you have described I am not sure your son would be ready to step up to something like this. This is not one of those jobs you can just quit if you get tired of it.
I am sending prayers for you and your son and hope things work out for him in his own time.
(((hugs)))