I don't think all flirting is a sin. It sounds like it is the type of flirting he is doing that is making you uncomfortable? I would just talk to him about it. His response will tell you what type of guy he is.
That is hard. I would still go to church and tell yourself it is okay to step out if he calls. I do remember a while ago running late to church and then my husband called when I was in the parking lot. I ended up just sitting in the car talking to him. I missed church but sometimes that...