Honesty. That's the only way to go about this. I speak from experience. My ex said he was all ready to be a step dad, but he was faking. We were together for 7 years, and those years were horrible. He didn't like kids in general, much less mine. My kids fell in love with him because he was faking in the beginning. Their hearts were broken when he couldn't "act like a dad" anymore.
Please be honest with him for the sake of the kids, and yourself and your partner.
Don't stay in that situation out of obligation. You're running head first into a painful life.