Welcome to the forum. We are a friendly , caring and supportive community.
I can see how confused you are by kissing your best friend, as you thought you had one type of relationship and then it seems to have change.
I had no sisters only brother and had more male friends and than female .
I have had experience when a best male friend has turned into a romantic friend but for me that didn't turn out very well and I also lost a friend. However for some friends things do turn out and a friend of mine married her best friend.
The thing is once you turn it from a friendship to a romance it is very hard to back to being friends.
I think it may help if both of you talk openly and honestly about what you want and do not rush into anything.
You said he is unsure of what he wants and I assume you are young, I suppose both of you are unsure and a bit worried about what direction your relationship will take.
If you do decide to have a romantic relationship and it does not turn out , who would that make you feel if you lost him as your best friend?
Feel free to post here as much as you like and let us know how you are feeling.