If he is really interested in you, then he must see something in you that you don't. I mean hell, I used to believe any guy who would want to date me must have been crazy because I didn't consider myself attractive. Even now, I'd rate myself as decent strictly by looks, but not exactly supermodel material. If he is serious about keeping a relationship with you if you guys start dating, then he'll make sure the girls are made aware he's taken. Just because he's popular doesn't mean he'll become a playboy. All I can say is don't try to let on how bad you feel about yourself, or it can become very off-putting. Try to find positive things that you like about yourself and focus on those.
Have you tried asking him what his impression of your relationship? This is a question that is safe to ask usually, as it is an important one for people who are just friends and more.
Don't fixate on looks much. Trust me, when you get to know someone, appearance takes a back seat. Most people, once they get to a certain mental age, understand that it doesn't matter how pretty they are, if you can't stand their company, your relationship is doomed. And it certainly sounds like he doesn't hate your company
I also completely understand how you feel. It's scary. You are worried you aren't good enough, maybe are totally misreading the situation... The scariest part (at least for me) is being scared you'll lose the friendship you have with him if you tell him how you feel.
Sit down, have a heart-to-heart with him. He sounds like he'd be understanding enough to hear how you feel and not get too freaked out.
Yes, that's exactly it...no matter what, I'd like to still have him as a friend. It is very scary. And he is very understanding, he's helped me through things several times before...I will try talking to him. Thank you so much I feel some better now ^^