If you don't want to sit by him, then don't. You can always tell him that you will be happy to discuss son's birthday party another time. You don't want to be discussing things like this in a crowded place where others can hear what you are saying, especially if one of you gets a bit heated about something. Leave the party on the table, go and enjoy the concert this evening and keep the focus on your son and his concert.
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.
