Why is losing a game a big deal?

Because he's a bit of a perfectionist as many autistic people are. Can I assume we are talking about board / card games? It's like completing the pattern. Suppose I give you numbers with gaps. say 1, 2, _, 8, 16, _, 64, _. And you start to fill in the pattern and so you get 1,2,4,8,16,32,64 ... and then some *** comes in and writes a big letter A or the number 5 or anything other than 128. Can you not see how irritating that is? But in a game trying to meet the win conditions for the game is a bit like trying to complete the pattern, only then someone comes and ends the game before the pattern is complete.

I've said it before but autistic people make very natural problem solvers because we're quite driven to solve problems we care about. How to win a game is one more problem to solve ...

What can you do? Do nothing. Frustration is part of life. He needs to get used to the idea that sometimes you have to keep working hard to win and solve your problems. All you can do is comfort him and remind him he will get better with practice.

Oh! I love this. I didn't love the frustration growing up, but not having met with so many doses of it, I wouldn't have worked harder in every conquest. 

JuniperFromGallifrey Can I ask as a parent, what would be the best way to support a child through his frustration in situations like these?

Great advice JuniperfromGallifrey. Thank you. 

Great advice JuniperfromGallifrey. Thank you.