You can ask him why he changes so much, what's his idea, maybe he'll share strategy.

When someone changes a lot, I would play a formation with flexibility. For example some mangers like players with 2-3 positions so they can switch between maybe 451v and 4141 without substitutes, just the same 11 swapping positions.

For myself, I use 4 defense, 4 midfield 1 striker and either AMC or DMC depending on the match. Pretty balanced overall. During the match I use tactical changes based on player's ratings for both teams and the statistics, which update often by returning to the home screen and reloading the match quickly. The changes are just based on experience, notes I've taken, etc.

Look for what's weak and strong at that moment. If a manager arrives mid game, I might have to use possession bonus instead of attacking. If I have the lead and they start putting more players in my side, maybe i need hard tackling, or high pressing with hard defensive passing to waste time.

Don't be afraid to make changes, even just for a few minutes. And don't panic if you see a change, just consider if that changes strong and weak points