Your son is a teenager, he has a girlfriend now and that's the most important thing in his life. Once your precious little prince/princess become teenagers, all bets are off - parents take the back seat. That's how it is with every one of us parents. They just want to be with their friends and not spend any unnecessary time with mom or dad.
I am sure your ex-wife doesn't see your son either, he has better things to do. Children play their parents for their own selfish reason to gain whatever they want. It's unfortunate, but your son isn't an isolated case here either.
My advice to you is - keep the x-box wrapped in your house and wait until your son comes around. I bet he'll come right around Christmas day to see what you gave him. Perhaps it will be a good time then to speak to him openly and frank. Tell him that you feel abandoned by him and that you would like to see more of him and if it means inviting his girlfriend too, then do so.
Be patient!! That's all you can do with teenagers - be patient!