FWB Situation

My FWB constantly flirts with me (I think, I’m not sure what flirting looks like), stays up texting/talking to me until super late (or super early), and lastly some of our friends have started to notice that there might be something there. I guess my question is how do I know if he’s flirting and how do I approach him or start the conversation about maybe making the situation more serious without freaking him out? Please help! I’m super oblivious when it comes to boys!!