This sounds like a really tough situation to be in, I’m sorry!
I’m not sure if you still need advice, but I can kind of relate to your situation. My best friend in high school and I spent a lot of time together, and I was totally clueless that she wanted to be more than friends. I didn’t view her as more than a friend though, so we never ended up dating…
It’s been a couple years since we graduated (we both go to different colleges now) and I don’t think I have feelings for her yet? But she’s found someone else that makes her happy, and I’m glad about that! But I haven’t met anyone else yet so I’m still not exactly sure if I made the right choice lol. And I still enjoy hanging out just as friends once in awhile — maybe that’s the whole “absence makes the heart grow fonder” think idk. Love is VERY complicated, that’s for sure 
If you haven’t yet, maybe try asking him what he exactly meant by insert flirty message here. Hopefully you can both communicate your feelings about the situation! If he doesn’t seem to want a romantic relationship, then he’ll know better that sending flirty messages probably isn’t the best idea. But hopefully you both feel the same way about each other and there will be less pressure and anxiety after talking it out 
I really hope things work out for the both of y’all!! 