How to let him down very very gently?

I need help. I'm a senior in high school and his an incoming freshman. We met before at my friends house (that was a while back) then at the begging of my summer break he started messaging me that he likes me and all this stuff. He's going to be playing football this season at our school and I'm an athletic trainer there so were going to see a lot of each other.. I don't really want to go out with him because he really is young; and he's a cutie so I know he'll find a girlfriend in no time. But I want to know what kind of hints do I drop to tell/show him that I kinda don't want to go out with him. I've tried telling him his to young and he's just not getting that.

anything you give me is greatly appreciated. (any sarcasm isn't take that bs elsewhere.)

so please help I don't want to hurt the young one to bad.

Thanks :)))
How to let him down very very gently?
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