Guy's Behavior

How can I be 100% sure a male friend is interested, and how can I express interest back?

We're both in our late 20s and single. Met as coworkers (he is my superior at work). We became friends and frequently see movies together & have gone to dinner together a couple times. He flirts with & teases me a lot and touches me (not in a weird way lmao) & tells me super personal stuff, calls me different pet names, frequently looks for my approval/input on things, once sent me a photo of him sleepy in the morning (shirtless, I may add, could only see shoulders though), and has told his friends and family about me. The last two times we've done something together, he hasn't told our other coworkers about it like he has done in the past which made me a little suspicious. I've never had male friends so I don't really know if this is how they act with female friends. If it matters, he has only sisters and in my experience men with sisters are more likely to have platonic female friendships. That being said he only has one other female friend who is twice his age and he views her as an aunt. If I'm right about him liking me, then I do know he hasn't made a move because of our jobs, as the other day at dinner he randomly decided to bring up that dating coworkers is "trouble".

However, he's leaving to work at a different location soon since he can get a promotion. He called me to tell me about this (before he even told his mom) because he knew I'd be upset. I'm heartbroken I won't be working with him anymore and he lives so far away from me too so work was kinda like a mini-hangout.

I'm taking him out to celebrate soon. We won't be seeing each other as much anymore, and I just have a bad gut feeling that if I don't say something then this relationship might fizzle out, especially since we're both people who like to communicate in person rather than text/call. Mainly I'm scared that it's not reciprocated and that I'll lose him as a friend if I confess. Is there a way I can show I'm interested without being too insane? I need him to make the first move unfortunately

How can I be 100% sure a male friend is interested, and how can I express interest back?
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