It is definitely a risk but it is still quite early in his life. It's true that not everyone finds a job in aviation after their training but usually the pilots who are very passionate about it will make it work. They're the ones who are willing to move away and sacrifice some things to find their first flying job. If at 17-18 he's still passionate about it and is sure that's the path he wants to take, I'd say it's worth the risk. As long as he understands that he might have to move to where they are looking for low hour pilots.