Yeah, you need to fess up sooner, rather than later, but this isn’t as big of a deal as if you’d been 16 or 17 and lied about being 20. And, even if he decides it’s a deal breaker, better to know now than invest more time into this and deal with the potential fallout of your “secret” coming out down the line, like, say for instance, your supposed next 21st birthday (if you’re in the states).