Need help with an oddly specific insurance question.

Hello! I was just redirected to this subreddit after asking this question elsewhere. Hopefully you all can help me figure this out.

My fiancé and I are looking to elope sometime soon, but the issue of insurance popped into my head. We’re looking at elopement because he doesn’t want his mom to get involved or interfere with us getting married.

For context, my fiancé and I currently live in Florida with his mother and stepfather (although we are hoping to change that in the next several months). My fiancé is 26 years old and is currently still on his stepfather’s insurance. His insurance will allow him to stay on it up until age 30 at most (circumstances are re-evaluated annually in that regard). I’m a medically retired Veteran and have Tricare, so to add him to my insurance, we do have to be legally married.

My question is: if I go ahead and add my fiancé to my insurance, does he need to be removed from his stepfather’s insurance? And at what point would this need to be done? We want to avoid letting them know about our (soon-to-be) marriage for as long as possible.