You announce it when you are ready, I told family and close friends and waited to announce of facebook.
Do it when u are comfortable. Thats really sweet and awesome that hes so excited about it but u are going the one actually carrying it. I would say though tell family and close friends before anything on fb though cuz there will probably be some "this is how I find out?..." my mil still throws it out there how she was one of the last to know about my 1st son from and that was almost 3yrs ago