@zensky I applaud your desire to encourage him to be intrinsically motivated, and to avoid using extrinsic motivators of a positive of negative type. Education theory and research tells us this is the best way to go.
I think you should be (and probably are) proud of his achievement in national fitness championships (I have no idea what that involves—is it like gymnastics?). His way may not be academic, and if he is taking after his father, then you must surely feel you understand him.
I would encourage you to come clean to your son. Tell him you were like him. You didn’t do well in school. And this was the consequence in your life. And this is what you did later in order deal with it.
Do not see this as a cautionary tale, though. See it as showing him another way of doing things. I think you probably want him to feel good about himself and his potential. I don’t think there’s any need to make a kid feel bad for not doing something. You are much more likely to get them to do something by focusing on what they do well.
So let me ask you. What is your story? Or rather, since you don’t like to share your story in public, to think about your story. What did you do after high school? Did you go to college? What was college like for you? How many years did it take to get through it, if you finished? Then what did you do? What was your career like? Did you ever go back to school or get any further education? Why? How was that?
You want to show him what your life was like and what you learned, simply as an alternative. It is not necessarily a lesson. Show him you trust him to make his own decisions. Really, you have no choice now. He knows what he likes. He needs to figure out how to move forward in that area. Perhaps he will become a fitness instructor, in time. Perhaps he will teach. Perhaps he will open his own school. Perhaps he will compete.
Whatever he wants, he needs a plan. Be open to any plan and be open to helping him plan.
Of course, you may have some disagreements with your ex-wife. That makes things difficult. But you have to give him the best you have, and I think you owe him the truth about what you think and who you are. Do not let her come between you and your son. Don’t let anything come between you. Your relationship is the most important thing—more important than pretending to be someone you may not be, because you think that will encourage him to be more academic.
He is who he is already. Relate to him on that level. Not on the level of who you want him to be. I believe you will be of more use to him by being honest than by trying to maintain some notion of what it means to be a parent.
I could point to myself as an example of that, but I’m sure there are millions of others who do the same thing, and I bet you will feel more comfortable that way, anyway.
It’s ok. You are a good father. He will do well. Just help him find his way.