I would support him, and help him plan to take each step to get closer to his dream. Realistically, pursuing a dream like that is going to get him farther than not pursuing it. Even if he only manages to get his GED or only manages to graduate from community college, that's still much further than he would get otherwise. When you tell him that his school is going to help train him for a job, make it clear that it doesn't have to be the only job he ever has - but he's very lucky to have a school that will help train him for his first job! And the fact that he will be able to get experience in the medical profession is an added bonus that could help him one day in applying to medical school.

But I think he also has another issue that you need to address - he feels that professional achievement is a sum of his worth. He thinks that being a doctor will show that he is "good enough". This isn't a good way for anyone to look at the world. There are good janitors and terrible doctors, and from anyone who has experience of these things would take the good janitor over a terrible doctor 100 times out of 100. The thing you need to teach him right now is that as long as he is a good person and treats others kindly and works hard, you will be proud of him no matter what he does. It's very important that he not lock himself into wanting to be a doctor so much that he is devastated at discovering at any point that he thinks it isn't the right path anymore. People change over time - look at Ken Jeong - he became a doctor, only to decide it wasn't the right fit and to become a comedy actor. If you don't build into your mind a certain amount of flexibility, you can't deal with a change like that. One of my roommates' boyfriends told us for years he planned on becoming a doctor. When he graduated college and didn't get into med school it destroyed him.

But as a person who was discouraged by my parents from taking certain paths, even though they were occasionally right, I not only didn't appreciate it, I still hold it against them. My mother lives in fear of my telling people about the time she told me I couldn't become an actress because I wasn't pretty enough (I'm not particularly pretty, but in retrospect it wouldn't have been a problem if I'd actually gone for it - attractive is not the dividing line for working actor).

And then lastly, I am going to point out that with the medical technology in the works, doctors could easily be mostly obsolete in the next ten years (with dramatically fewer needed) So your son's ambition could change, too. He's not going to remember how he gets from point A to point B career-wise - but he is going to remember how you make him feel.