Why would a freshman in college want to talk to a freshman in high school?

You should watch out because you're right, that is unusual. Because when he was a junior in high school you were in 7th grade. That is very weird and he is probably looking for an easy person to get with. Not easy, like a tramp but easy as in you're young and naive and he knows this. There is a big difference between a freshman in college and a freshman in high school, I know, because I've been both. Tread lightly in this and don't take the relationship seriously, because it's probably not for him.