What should I do in this situation?

I know this sounds petty but I do not know what to do.
I met this guy 4 months ago and I REALLY like him! I have never felt this way about someone before. The problem is that he has a girlfriend, but when he tells me about her it sounds like there’s nothing there anymore.. I think the reason he is hanging on is because they have been together so long (almost 2 years). I know I should stay out of it and do nothing, and I usually would except in a week I might never see him again. I don’t know him deeply but as soon as I first met him there was something, I cannot describe. What should I do?