I'm officially off birth control and my boyfriend doesn't know.
So now you're stopping? Why did you start birth control in the first place?
Because I had a serious medical condition and we thought that it was impracticable to get pregnant. And he didn't want kids yet.
Why aren't you telling him about your decision? If he doesn't want kids, it's unethical for you to deceive him.
It's fine. If I get pregnant, I'm sure I can convince him to stick around!
Keeping secrets from your boyfriend is generally not something you should do. It's especially bad when it's something as significant as being on birth control. Your boyfriend has the right to know that you are no longer taking birth control so that he knows about the increased risk of you becoming pregnant.