Is it worth it? He is 19 (I am 25) with a girlfriend, but I just know there is something there.?

So a few months ago, this guy started to work at my job. I fell for him immediately. 2 problems. He is 6 years younger than me and he has been dating the same girl since 8th grade. The age difference I got over but the girlfriend thing really bugs me. He is the sweetest guy ever and we are becoming great friends and I swear he totally acts like he really might like me back. Sometimes I think that maybe by my telling him that I like him (yes I did) that he is perhaps maybe rethinking things with his girlfriend? Either way I am totally falling in love with him more and more every time I talk to him. What do I do???