My boyfriend came inside me.. Advice?

Almost a month ago my boyfriend had came inside me while we were doing the dirty. I took Plan B the day after without him knowing.. Now he thinks I'm pregnant. 1. Because of "signs" he has been experiencing. And 2. He doesn't know I took plan b.

I was off birth control for 2 months and was supposed to restart it this Sunday but didn't because he's 100% certain that I am.. I promised him I wouldn't start it. What do I do?