No 2X4s from me today. The discussion of the cost of the prom dress needs to be discussed and what better what to do it then over coffee. Keep your discuss on the dress and very mutual topics and no relationship talks no matter what. If he asks a question or your opinion on something, then answer it. If he brings up a discussion about the relationship, say something like this "h, I'll need to think about it for a while, but today, let's keep our focus on the prom dress for our daughter. I'll get back to you on my thoughts about XXX later on". You only have control over you and how you react/respond to him. You have two choices, i.e., to discuss relationship talks or not if he brings it up. I would go for "not". This coffee meet up isn't about that, it's about the dress.
One last thing...keep your expectations very, very low. Try to look at this coffee meet up as a short visit w/a distant cousin or distant friend. Also, put that rubber band on your wrist and snap it if you are tempted to say something that you know may not sit well w/him at this time.
Good luck! Once this meet up is over, go NC for a while. Allow him to come to you.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.