My son thinks he has to win at everything he plays: Advice?

So, my son almost five years old. His cousin almost eight years old. She is the first child that my son knew about, and he loved her. He always wants to play with her. On the other hand, when he plays, he wants to be the winner always ( I know we are working on this little by little), so they play, they cry, she wants to win, he wants to win. Sometimes she let him. Lately, she doesn’t want to let him, which we all respect. My son asks for her a lot we all kind of agreed not to encourage them to play together because of the age gap that will make my son the loser, which he doesn’t like, and it will end with crying. What do any of you do? How to handle it? Of course, due to covid not able to find him friends