What should I do about him?

Well, that depends on how much you care about him. I doubt this is love, because I for one believe that (for the most part) you don't really love someone unless they love you back aswell. Of course there's some acceptions to this theory...but anyway.
If you love him enough that you truly do just want his happiness, then let him have that girl. You'll deal with second place for his happiness right?

If you care about him enough that you like him, but want him to like you too and that's it. Then you have to talk to him. Let him know how you feel. I know it'll take alot of courage, because you might get hurt and it might hurt him. But if it's what you really want then it's you're only choice. You can't just watch him go to the other girl; because if you do want him that badly, then you're only going to get jealous and start to throw around your anger of the situation at the both of them. It's not a pretty scene.

Your last choice is just to move on from him. If you respect yourself as much as one is suppose to, then you won't take a backseat to anyone else. You should want to be with someone who wants to be with you. So leave him be, and let your eyes wander.

As long as I'm being honest, I don't think any of my ideas will help you decide at all, because they all have good and bad outcomes. Not only that, but I've found that no matter how much advice I get, no matter how much I might try to think for myself, and no matter how much thought goes into it - I usually don't do anything about my problems at all. - But this is my flaw, and I hope that you are stronger than that... Don't stress yourself out over it. Life goes on. I wish you the best though.

just do wut YOU want im tired of girls thinking that they need to do things for the guy they like. if he doesnt come 2 u u come to him. tell him. let it out... dont just let it go.

i kinda agree with the two comments above... because they both are making totall sense...

in away i think yeah if u care like u say you do then best thing is to let him be with her and when it goes bad youll be the one who picks up the pieces.... and youll be the one he knows he can rely on....

but then if u dnt spk up to him then he wont have a clue about how you feel,.... but thing is with doing tht you can push him further away and WHAT EVER you do dont bad mouth this girl because it wont get you no where apart from pushing him closer to her and further away from you!!!

but i tihnk you should sit down and think thid thro and decide is it best to leave him alone to be with her and be a gd friend he knows he can be rely on you when he needs you... or do u tell him how you feel an take the risk that you might actually lose him!!!????!!!!!

like that famous quote we all have heard

"JUST BECAUSE YOU LOVE SOMEONE THE WAY YOU DO.. DOEST MEAN THEY LOVE YOU THE SAME WAY BK!"

take care and good luck..
fingers crossed it wks out for the best
xhx

You do not love him, you merely like him. That is why you become so arrogant, and care only for him if he is with you. [No offence intended]

That is what I feel for a girl. I want her to be happy, but only with me and noone else. And it is not a nice feeling... to be this arrogant.