i would say it’s worth just ripping the bandaid off like #1 said and asking him about it. he might be questioning his sexuality and see you as a safe person to do that with (whether that’s fair or not). he might be romantically interested in you. he might just have drunkenly made out with you. i can’t read either of your minds. nobody else here can either. the best way to know what he is thinking and feeling is to ask him about it directly, without making it a confrontation.
just be sure to give yourself grace and also time. kissing your best friend is bound to stir up a lot of strange feelings, even if you aren’t attracted to him. writing those out might help, or talking them out too if you have a safe third party to discuss this with. don’t try to force yourself to feel one way or the other. it’s natural to be scared of this conversation because it could change your relationship, but he’s your best friend for a reason. just try to keep an open mind. good luck!