I have a 16-year-old son. I’m in my early 30s (had him very young) and a professional footballer. My son also dreams of becoming a successful footballer (he’s been playing since he was 6), but he’s just… not great. He’s good, but not great - and in this extremely competitive industry you need to be at least great in order to even stand a chance. So I told him, as someone who’s been doing this for a very, very long time & is active in this sphere, that he should find another, more attainable dream. He took it as me not believing in him, but I’m just objective and realistic.
You should have known that there was nothing to gain by telling him what you did. Kids that age are smart enough to realize that if they aren’t being selected to the local all-star team, it’s because they’re not an all-star. If they go to football camp and they aren’t one of the best people at the camp, they’ll realize that they’re not very likely to go pro. But you decided to make it your business at a time when you didn’t need to, and that makes you a jerk.
You said that you’re just being objective and realistic, right, but you decided to tell your son your opinion, and not someone else. If you were actually trying to be objective, you would have told everyone on the team what you thought about their potential. Of course that would be really rude, which is the point.
What you could have done is what many other people have mentioned in the comments. Something about how there’s no guarantee that anyone can make it pro, or how long they’ll last if they do, because random injuries can end your career, and the median length of a professional footballer isn’t very long anyway, so there’s still the rest of life to live.