Please help me help out Lukas?

LAST year I liked this really sweet, shy guy Lukas. He's really smart but a jock too, whereas i'm a shy, artsy nerd. I've known him since 2nd grade but didn't even notice he existed until 8th grade because we're both so quiet and different. He was always into soccer and over the summer he got really muscular & hot from it. So now he ACTS really vain, pervy, and otherwise a jerk, always looking for attention. I go to a different school now but i saw him a couple of times since graduating and i was sooo disgusted! He's totally different! He stares at my friends' chests and is loud and annoying. I really want to help him, but idk what's best for him and I don't know how to help him even if it is best for me to try to change him back to how he was. I also don't really want to hurt him or yell too much, as we're both shy and not really friends. I might only see him 6-7 more times in my LIFE so I don't even have much time!!! I REALLY owe him because of a long story where he basically made me realize what an *** i was, so now it's my turn and I can't just let this go. He sort of isn't a complete idiot YET. It's like he's trying to be popular but inside he's still sensitive. See, some of my friends who still go to school with him don't put up with him so when they yell at him for being pervy he gets upset because he has a crush on one of them and she USED to like him back, but now she hates him because of the above said. He's still really sensitive inside and I think he's just acting like this to get popular, so I think there could still be hope but idk. I don't want to make things worse! What should I do?!? Please don't tell me that i can't change anything, especially cuz i think he's still the same guy inside. I REALLY want to help him out. i need a subtle way to help him out without being too awkward and without him hating me too much. To make matters worse, i blush and spaz whenever i try and talk to him so he gets really weirded out. please be specific and realistic about what i should do?? And is there some way to help him where i can take advantage of my shyness, like act really sad when he's pervy so he feels bad and stops? or someone suggested writing a note? This isn't who he is. THANK YOU!