Whether or not his relationship with his girlfriend is serious, it’s pretty messed up that he’s willing to do this to her after a year of being together. If you like someone enough to be involved with them for a year, it’s pretty disrespectful to kiss someone else for a month. Sure, he could do it to you in the future. The real issue is, do you want to involve yourself with someone like that? You know he is cheating on his girlfriend with you. It would be more self-respecting to stop seeing a guy like that, but maybe your feelings for him will win out. You should definitely think about it.
Then there’s the fact that you work together. If you’re young, and it’s just some after school job that you don’t really care about, it doesn’t matter too much. If this job is how you support yourself, it’s something to worry about. Dating someone you work with can be tricky under the best of circumstances. This guy has been seeing you and his girlfriend for a month, so it’s not like it’s some impulsive, one-time thing- he’s intentionally doing this. If he has it in him to do that, maybe he’s not the nicest guy ever. What if things go wrong in the relationship, and he causes trouble for you at work later?