Welcome to HeartSupport. You’re in good company here.
It sounds to me like he is flirting with you. I don’t think you’re wrong about the vibes you’re picking up. That kind of attention feels excellent. We were bred to seek human connection and intimacy. You’re feeling that from him, and I’m sure he’s feeling it from you too, so you keep talking and doing nice little things for each other. It feels really really good, and that’s why we do it.
That said, it’s not necessarily a good thing, especially if he’s in a relationship. Maybe his relationship is failing, or maybe it’s just become routine for him, but either way he’s seeking something in you that he doesn’t have with his SO. Maybe he wants that new-dating feeling again, because it’s really intoxicating and doesn’t come back when it’s worn off, or maybe he wants a fresh start with someone who doesn’t see his faults. You are not the guilty party for grabbing his attention from his SO, that’s on him. What I will say though, is if he’s happy to flirt with you, he’d be happy to flirt with someone else if the two of you eventually wound up together.
Members of couples will meet people who make them feel that tingle, it’s inevitable. The difference between a steadfast and an unreliable partner is whether they turn from or pursue the people who make them feel those feelings. I’ve been on both sides of this situation. It is SO hard to turn away from someone who makes me feel fresh and newly desirable, but I know even if I try to keep things casual with someone like that I’ll eventually get emotionally attached. On the other side, I started dating a girl who left a lukewarm relationship for me. She rocked my world, until we got into the comfortable routine or a more serious relationship, and then she left me for someone else new and exciting and devastated me. When I finally got over my bitterness, I figured out she wasn’t trying to play me and she really believed we had something special, but she was looking for fireworks, and fireworks are unsustainable long-term.
One more word of caution, getting involved with coworkers is very risky. In the US, there are laws protecting workers from inappropriate workplace relationships. From HR’s standpoint, the only thing between a workplace relationship and a lawsuit is one person getting hurt and deciding to say they were treated inappropriately by the other. Also, if one of you is in a higher position than the other, that could be seen from the outside as favorable treatment for a romantic partner. Either way, to get ahead of it, HR might reassign, reprimand, or even fire one or both of you if they think your relationship is inappropriate.
I hate to be a wet blanket. I am happy that you feel loved and valued, and in any other scenario I’d say congratulations! There are just issues that could make this one potentially dangerous to your emotional health and job security. That said, if you wanted to know for sure, I like your idea of telling him that you’re thinking about leaving because of how you feel about him. Just be ready to either follow through with it or swallow your disappointment if he doesn’t return the sentiment.