Its fine.
It will be fine.
Your son will probably do fine with the small class size. It is a positive thing.
My daughter was/is shy too. And when she was young like that, she did better in smaller classes. Which the Teacher even said so. And I agreed. Because I knew my daughter.
As she got older, my daughter naturally got less shy. But, being 'shy' is NOT a bad thing... my daughter is actually very perceptive and wise and self-confident for her age.. .and discerning. So I never treated her 'shyness' as a negative thing. I nurtured her. She is also very socially cognizant... more so than other kids her age or older.
I think, there is no problem with your son's small class size.
all the best,
Susan