I think you are being very considerate of his feelings, which must be tough given that you are unhappy enough to not want to stay married. I'm sure this is quite an effort for you.
So you have known for quite a while that this marriage was over, but he has not. You have essentially processed this and dealt with it before you officially separated, he is just beginning. It's tough when you are both at different places in the process.
On one hand, it is wonderful that you are not being a jerk to him and hammering him with boundaries- this will come. On the other hand, he is likely talking about the best friend thing as a way to stay as close to you as he thinks he is now, in other words just renaming your relationship without anything having to change. What is harder to tell is if he will begin to deal with reality and adjust this thinking or cling even harder to it once reality really sinks in. You would know best.
I feel like it's ok to start kindly talking with him about how you hope to be able to have honest conversations about what the post-divorce relationship will look like. Dropping hints that you won't be going to so many events together and having him 'meet you there', etc. This will be your biggest clue as to how he will handle things as the process moves forward.
And you are not doing this because he necessarily deserves this kindness, but because you are going to have to co-parent with him and the better that relationship, the better for all involved. I don't think you need to be so blunt yet, I really don't. I think give things some time and see how he is adjusting- dealing with things or becoming more clingy. But now it is quite new for him, cut him some slack.
I had a friend who divorced her husband at her request. He did not want the divorce at all. She felt really guilty because he hadn't really done anything wrong (her admission, she admitted that she just found him boring and wanted more excitement at mid-life). So when he wanted to stay really close, she did for the longest time. They went to lots of events and parties together, and he was pretty happy. Then she started dating- he was crushed. They had never sorted out how things would go and what boundaries they would have. Once she got remarried, he did get pretty angry and it made parenting tough. I know this is different than your situation, but just to share someone else's experience... it was nice of my friend to give her ex some time and kindness, but she never went further when the time was right to have the divorce look like she wanted and needed.
Good luck!