What do you do?  Nothing other than be appreciative of having spent an enjoyable day with your brother.  
 

If it were an actual match, he would have withdrawn due to injury.  Nothing at all unusual.

It’s time for you to introduce him to the rules and etiquette of golf before this becomes a habit. The sooner the better. Sure, no big deal if he is only planning to be a casual golfer,  but if his intention is to eventually make it on the team, he needs to be honest about his scores and start practicing telling the truth now. 

31 minutes ago, FlyingAce said:

It’s time for you to introduce him to the rules and etiquette of golf before this becomes a habit. The sooner the better. Sure, no big deal if he is only planning to be a casual golfer,  but if his intention is to eventually make it on the team, he needs to be honest about his scores and start practicing telling the truth now. 

Exactly what I was thinking. He may hate me for it for a while, but if he’s gonna be on the team, he needs to start playing by the rules. 

Tell him the same thing we'd tell teammates after practice when they shaved their scores down when they played a shitty round.

103? 103? that sounds about right if you stopped counting on the 15th hole

Very easy fix.  You place a $2 bet at the beginning of the round.  You state that if you don't finish a hole you are carding a 10 for that hole. No exceptions.  If he hands in a score that is unreasonable after a hole, you stop and ask him about it.  Simple. "Wait, a six? You hit one in the woods on your Tee shot right? and then you took 3 additional strokes to get on the green, and as I recall you triple putted.  That's a nine, right?".