How to play 1 vs 1 game with my son?

Only works if playing on same system. First turn off internet connection. Then in settings, go to edit teams. You can then transfer any player to any team. Then you can play modes in kick off without online squads being automatically downloaded. Then,in the start match screen,  edit your squad, formation etc. The transferred player will be the last one on reserves. Hope this helps.

No, it’s almost impossible to couch-play anything else than default squads and default controller settings. Will result in game crashing if u try something else. 

I haven't personally experienced that on PS5. However, with the amount of bugs I have come across, I wouldn't be surprised.