How to politely let him down?

I have a good friend who seems to have romantic feelings for me. So I actually originally met him via work because he was one of my former employees.
After a couple weeks of him working there, he had quit due to scheduling issues. After he was terminated, he started making romantic/flirtatious moves towards me. I didn’t really know how to respond other than ignore, I thought it was kinda inappropriate considering I was his manager, but I still wanted to be his friend. At one point he texted me saying that he had a crush on me and then proceeded to ask me on a date. I politely texted back saying that I was down to hang, but only as friends and wasn’t looking for a relationship. When we did hang, I did finally tell him about my partner (who is long-distance). He was really cool about it, he even liked a couple of pictures of us that I had posted on social media. However, the inappropriate behavior hasn’t really stopped: he’ll sometimes text Very flirtatiously in the middle of the night/super early in the morning, or sometimes spam-like a bunch of pictures (that are old), or call me honey or sweetie or cutie or doll or other affectionate terms. Ironically, he’ll also like/comment on posts my partner tags me in, or will leave positive comments on selfies I take with my boyfriend (if anyone can explain this behavior, that’d also be great!). I have tried to make more of an effort to casually talk about my partner, but it hasn’t really calmed down his behavior. At this point in time I don’t think it’s very healthy, I still want to be friends but I also think that he’s kind of disrespecting my relationship and my boyfriend. I don’t want to hurt his feelings because I get it, I’ve had crushes in the past that didn’t feel towards me and that made me upset. But I do want him to also stop and have respect. What do I do?
How to politely let him down?
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