You've come to a diverse pathway. Personally, I'm engaged, so I guess I can take the other side of the standpoint first:
I have best friends. My best bro' is named Hunter. I've known him as long as I've known my fiance, but, she has priority over him. She had priority over him when we weren't engaged. The reason why? Is that a relationship is a responsibility. Sure, friendships are responsibilities too, and you have to pay them attention as well. Thing is, in a guy's mind, especially if he's young and doesn't know better at the time, the ultimate pay-off for most is just getting laid.
Sounds crude, I know, and I'm sure there are gonna be some people saying "Oh, well I'm not like that!" Yeah... at one point or another in your life, you are. It's instinct. Primal instinct. So, your friend should still care about you, theoretically. Thing is, he's just more worried about getting some at the moment. Give it another week or two. Go ahead and invite him to your birthday party. Him only though.
That's where the other branch of this comes into play.
If you only invite him, and not her, then he'll have to choose. Making someone choose isn't always the best sign of friendship, but if you're THAT worried about him (which almost seems... odd), it'll prove which is more important. Simple as that.
It does bring me to my final point though:
Throughout your life, you're going to make friends, and you're going to lose them. Odds are, a friend you make in high school, you won't be talking to in about 7 years. Things like this happen, and it's a natural part of the social world. Get used to it, and enjoy what time you have with someone already.[/QUOTE
This is.... really good advice. There are a couple of points in there I' don't quite agree with, but that's just me. Overall, this is good.