Two words: f**k and buddy. I think he wants you to sleep with him, hopefully more than once, and hopefully on a regular basis. He's not lying when he says he doesn't want to date you, at least not in the way you probably think of dating.
If you really think he wants to "grab drinks" with both of your significant others, suggest that and watch him find reasons not to do it. The suggestion is a smoke-screen; he wants drinks with you and then a good tumble in the sack. Without your boyfriend or his girlfriend.
If you're happy in your current relationship, tell your coworker you are taken and ask him politely to stop hitting on you. If you're not happy in your current relationship and you want to sleep with this guy, at least give your boyfriend the courtesy of breaking up with him first, rather than cheating on him.
Still, I do not recommend sleeping with workmates for numerous reasons (see previous answers from several of us about this question). It's a tricky business and rarely ends well. Also, if I'm right about this guy, he's a cheater and you should avoid him.
Thanks for the question.