Ughhhh...wow, as a parent, I can only imagine how you're feeling right now Don
I know I'd want to say/do so many things if this were my own sons (and who knows, one day it might be), but there's a part of me that also knows the more we parents push, the more kids push back. I watched my sister push and push and push back at our parents in ways you would not believe (they too were divorced) and it wasn't until they both agreed it's time for tough love that she was forced to grow up. She still made mistakes and fell flat on her face; it was so hard to watch for my parents (sort of gratifying for me, I won't lie...it got old being the only responsible kid LOL), but for all of their "helpful advice" and trying they did, all she ever did was give 'em the middle finger and do what she wanted. In the end, before she finally "grew up and got over herself", it cost her a college degree that was right in her very hands, and she never went back to get that degree. A full ride to college, *poof*, gone, all because she thought she knew better than our parents did. It tore our Dad up, but that was the tough lesson she learned.
I think knowing what I saw first hand, if this ever happens in my own world with my own sons, I'll likely have to do my best to let them make their own mistakes and learn from them. It's the hardest decision in the world because even as I type it, I really wonder "Can I do that?"
I do think it's OK as a parent to voice concerns for your child's well-being, and let them know you're there for them, always. But, there's also a limit to what you can do and they need to know that. If your son is heading down a path that will cost him his job or if he's walking away from that EMT role he wanted, that might mean a path of poverty. He needs to know you won't be there to fix that/pay his way while he does whatever he wants, THAT'S the line, right? You'll have to figure out where your limits are and make sure he understands where the lines are drawn.
Life is a lesson we all learn as we go through it and make mistakes. Some lessons are learned the hard, slow way; my sister lived that. I think it's the toughest part of being a parent - watching your child make the mistakes.