I really like this guy but i think he has a girlfriend what should i do?

i have been seeing this guy off and on for the past few years. i think he may have a girlfriend he wont add me on Facebook..... know a tad childish to care but it kinda strikes me as odd. could this in fact be because he is seeing someone else? there is a girl who always has a photo of the two of them and i have a feeling they are more than friends and when i ask him he just brushes me off and tells me she is his ex and they are no longer together. but this kinda rubs me the wrong way. should i just go ask her? i no it could be an awkward thing to do,, it i have strong feelings about cheating and i could never do that to another women.. so what i'm asking is thoughts and a new prospective