Differences, Differences.
I know what you mean when you explain your little situation there.
I've known a couple of people who I know longer hang out with that become very popular.
Here's the thing: people mature differently.
He may have gotten into different cliques or friend groups that boost his popularity.
And despite what you may think, football position doens't really matter all too much. If I was a girl I'd not really date people on the football team; some tend to be really ****y, arrogant, and just jerks (not saying anything about you though.)
So don't fret about it too much, different people change in different ways. It's just a part of life that you can't change. And think of it this way; if he gets all of the attention, you may be the better person. Think about it: when he wants to date a girl, temptation will always be there with the other women, cheating may occur, or he'll be torn apart by decisions. And he might be extremely arrogant like he rules the school. Less-popular guys may just have the personalities they might lack. Just saying.