Hi, I'm Alicia. Thanks for asking your question - I'm happy to help you today.
It sounds very strange to me, so I can see why you'd be confused. If you've been in a committed relationship for 9 months, and there haven't been any red flags or problems along the way, then I can see only two real possibilities as to why he'd change his status -
1. (and hopefully this is not the case, but I just want to be fair to you and point out all possibilities) He might be interested in someone else that he's been talking to on Facebook and wants to give the impression that he's free to date whomever he wants so that this person won't be turned off by his status saying he's in a committed relationship or
2. He was trying to make some sort of strange joke, but that possibility sounds a bit weird to me. I don't see what would be funny about it - and I don't see what anyone else would see funny about it either. You're probably right that he didn't realize facebook would send you a message to accept or decline, but somewhere along the way he must also realize that you'd see his status if you went to his page.
It's not something that needs to become a huge deal if you do trust him and you think he really was just trying to make a joke -- and you could just let it lie and see how things go but if it's really bothering you (as I imagine it would be) then I would suggest sitting down at some point in the future to discuss it. It could just have been some silly mistake that he now regrets doing, so you have to listen to what your gut tells you to do. Good luck.