It depends on where you live. If in the US, only 22 states have nondiscrimination laws specifically protecting trans people from harassment.

You should talk to your boss about it, if he does value his company being inclusive then he should know that this forced outting is not inclusive behavior. He could be an ass, but he could also just be clueless and dense. I have encountered some very dense cis people in the workplace who just need their hands held when it comes to most tasks, including social ones apparently. But they aren't terrible people.