Do I tell him what I know?

I met a guy at my place of work. He gave me his number, we started communicating and we've been seeing each other for almost six months. I've never felt more secure than when I am with him. I know this is more than just an infatuation. I actually care for him, but there's one thing that keeps me from expressing it. I found out that he is still with his girlfriend whom he claimed he wasn't with anymore. From what I know they've been together for five years but it became long distance due to his job and they haven't seen each other since he left which is quite a while. He has no idea that i know about this. From what i have heard from him, their relationship seems to be rocky. I feel that if he really did care about her he wouldn't be so eager to spend time with me. We've spent more time out on dates and have only been at his place once so I know he's not with me for the sex... or at least I think so. He's asked me on numerous occasions what it is that I want from him but I just can't bring myself to tell him. Do I tell him how I feel about him or do I tell him what I know? Or do I do both? I need someone's opinion!
Do I tell him what I know?
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