A child who is putting academics first over a sport?
Please don't discourage that!
He sounds mature. He sounds like he knows his limits and has weighed his priorities. He is old enough to make this choice. He has already played football and knows exactly what it involves. He was good at it but that does not mean he must never stop doing it. Maybe he has changed as a person but you are not ready for him to change. You are looking to see if "there's something he's not telling me" when the something may just be, "Mom, I like the sport, but I don't want to be Joe the Football Player any more. I want to be Joe the Academic Guy. I'm redefining myself and you need to let me do that."
You, yourself, admit that you gave up a sport and regretted it. It's great that you are able to step back and see that about yourself and even greater that you realize your own experience may be affecting how you are feeling about his choice.
Please do not let your own dreams from your past get tangled up in his choices for his own present and future. Let him quit if that is his choice. He KNOWS what football requires and is choosing good grades over football. That is wise of him.
By the way, someone else posted that colleges look at extracurriculars so he might regret not having football as an extracurricular to list when he applies for college -- Uh, there are many forms of extracurriculars other than sports! He might do far better taking up academic-related activities if he's now focused that way. There are science competitions, math clubs, debate teams, engineering clubs that build stuff, volunteer opportunities -- a whole host of extracurricular activities that will be very appealing to colleges, probably more appealing than football. I would not let any notion of "he needs a sport to get into college" affect him, or you, in ANY way.