My son's account is a measely 1,000xp short of getting the Season 3 20,000 XP collectible!
I was excited at first... UNTIL I realized that there seems to be no way for him to get it!
The only season objective that he can get XP on is doing 8 more solo battles which isn't going to happen before they shut down the servers for the sew season update.
I didn't know if I was missing anything else that might grant me 1,000 XP. Thanks
I'm pretty sure they already shut mut down for server maintenance. So your son probably missed his chance.
Even if you can still get in before the reset. It's hard to say what would be the fastest way to get xp. If there's any available, a daily objective would probably be the fastest. If all your dailies are finished it's going to depend on where your son is on his other field passes and I really can't say without seeing those.
There will be plenty of ways to earn XP in Season 3 (programs, field pass, competitive pass). I wouldn't stress too much about not getting the collectible.