@mary84, why would you like him “to let people know that he’s taken and has a girlfriend”? What difference would this make to you, or to your relationship?
If you feel happy, secure, and confident in your relationship and in yourself, I simply can’t understand why you would care about other people knowing that your boyfriend is “taken”, as you put it.
Although you have said you don’t want to come across as “needy”, that you would prefer it that people who come into contact with your boyfriend online know that you are on the scene suggests some level of insecurity, on your part. I can’t see why else you would want to ensure that people he communicates with on Facebook know about your relationship.
If my assumptions are right and this is the case and you do feel a little insecure, this is your issue and not a relationship issue, and something you need to deal with yourself, and I don’t really think it’s something that you could fairly ask him to alter his behaviour for. He isn’t misrepresenting his relationship status in any way, and you’ve got no basis to assume that he’s trying to hide anything (as you said, he didn’t publicise his status when he was single), so I think you need to respect the way he wants to conduct his business online, and understand that he may want to keep some things private.
Of course, all of my assumptions here could be totally wrong, and I’m just going on what you’ve mentioned in your question. In any case, before taking any action, I would ponder the first question that I asked you, about why this matters to you, and see if there is anything you can do about that yourself.
In my opinion, I think relationships go down a fairly slippery slope when people start making compromises as a result of their partner’s insecurity, and not because that compromise is something that is reasonable and right. Insecurity isn’t something that should be reinforced by partners changing their behaviour in the hope of temporarily alleviating some “issue” or another, it should be something that people try to work on and change.