@buthowdoyouknow he. Is. Not. Your. Friend.
he is a creepy work colleague being overly familiar with you.
does he act this familiar with all men and women or just a certain few? If he is with others what’s the similar factor between you all?
he’s getting a kick from having this ‘thing’ with you. Even if it never becomes physical, Romantic etc.
it’s inappropriate for you and the workplace. It’s most certainly inappropriate to his partner.
stop engaging and responding to messages outside of work. If he asks just give non committal noises about being busy. Don’t share loads of personal stories about what you’ve been up to.
start making your tone more about work when he approaches you in the work place.
start being too busy to talk etc
he’ll eventually get the message and move on.
it’s basically grey rock technique.
or start asking him lots of questions about his partner. When can you meet them, where do they hang out. I’d contemplate getting utterly fascinated solely by their other half and see how they handle that. Maybe not actually… stick to grey rock…