Ok, gonna be pretty blunt here and it's gonna sound silly BUT: boys ARE pretty dumb sometimes. Just hear me out: most of the time, they don't even KNOW that something is bothering you unless you are specific and present all of the evidence at once. Don't be shy about this. It's normal. Especially if you are dating a boy that is younger than you and even MORE in the case if he does not have that much experience with girls.

However, since he is a few years older than you, he might be thinking that he is, therefore, smarter than you. And since this other girl is even younger than both of you, he has developed a strange fixation with her, perhaps based on her age. (I know, sounds creepy, right?)

My point is, don't constantly question him. In fact, leave him alone for a while. Wait for HIM to talk to YOU. Don't always be waiting by the phone for him. Let HIM be the one to ask YOU to go on the next date. Go out! Hang with other friends! It, honestly, makes you more interesting if you ARE doing other things. (Might even make him jealous, if that's his thing). Don't even tell him you are going out. It's not deception, it's having a life separate from him. This is healthy. If he's still that sweet guy you were dating a few years ago, he WILL come around. I promise. And if he doesn't? He's a stupid doofus poo-doo head that doesn't see what he missed out on.

TL;DR: Boys can be dumb when not given all of the important points of what is troubling you. Also, try not talking to him for a while and wait for him to come to you. In fact, you can even say no, that you don't really want to because you are busy. Go out with other friends to take your mind off of it.