A year has passed since the kiss and both of you have likely moved on. Bringing it up now doesn't necessarily mean that you have feelings for him or that you are trying to start something romantic. You can approach the conversation from a place of curiosity and a desire for clarity or closure.

You could try sending him a message or arranging to meet up with him, and explain that you would like to talk about the kiss from a year ago. You can acknowledge that it was a drunken mistake and that you don't have any expectations or intentions beyond having an open and honest conversation. You could also express that you feel embarrassed to bring it up, but that you think it's important to clear the air and ensure that there are no lingering feelings or misunderstandings between you.

Before anything happens though, consider whether or not discussing this with M25 would be respectful to your current boyfriend.