How long has it been since your concussion? I would be very conservative about when you resume skiing, which is a high risk sport for concussions. Once you've had one concussion it takes a much smaller hit to get a second. Since you had symptoms for a while after yours, I'd probably wait longer before skiing again. My ex had six concussions, I know a little about it, lol.
As for going to the beer thing - only go if you WANT to go to it, with a good friend, and won't be disappointed if you don't see ex or see him with someone else.
He may be trying to keep you in his back pocket as plan B, or he may be trying to live out his fantasy of a happy divorce "we're still friends". If he really misses you and wants you back he would invite you to go with him.
I wouldn't go unless it's what you want to do anyway, you have a friend to go with, and you can go with no expectations.
