How do I set boundaries with a workmate without making things awkward?

How do you respectfully tell someone to stop being touchy and to create some distance, especially in a workplace setting?

Context:
I have a workmate who’s also my college classmate. We’re in different departments now, but we’ve known each other since college and were part of the same friend group. Back then, we’d often do group work at his house, and I got to know his family too, his mom would even invite me to eat with them sometimes since I lived nearby. Because of that, we became pretty close in a very normal, friendly way.

He’s been in a long-term relationship for years. I’ve met his girlfriend back in college, and everyone at work knows he’s taken. To be very clear, I don’t have any romantic interest in him at all, he’s not my type, and even if he were single, I still wouldn’t see him that way.

Lately though, things have been making me uncomfortable. When he visits my department at work, he’s been overly close, touching my hair, jokingly pinching my arm, and often inviting me to have lunch with him or offering to take me home. I always decline because I want to respect his girlfriend and keep things professional. I really don’t want to create drama or issues at work.

On top of that, he often opens up to me about his personal problems, especially issues in his relationship with his girlfriend. I try to listen, but it’s starting to feel like too much.

I don’t think he’s a bad person, and I don’t want to accuse him of anything. I just genuinely feel uncomfortable and want to set clear boundaries but I don’t know how to explain this to him without hurting his feelings or making things awkward, especially since we have history as friends and classmates.