Lots of people talk about that SATs score is only for school benefit. But, for a child, if he/she has low score, it does mean he/she doesn't master all the skills he need. I wouldn't want my child to have low scores , not matter how it is for school measurement. And if the child doesn't master all the skills needed, there is a good chance this will affect his further learning along the way.
I agree Infant school is a bit earlier to learn seriously, in my country , we start at 6 or 7. However, this country , children start school at 4, it is unlikely to be changed overnight. If they don't master the skills taught in these infant school years, Junior teacher won't go back to teach them basics stuff learnt in Infant school. If parent don't do anything either, the children will stay in lower group.
As for some people think it doesn't make difference in a 30 children group or in a smaller group, that's why some people send their children to private school, isn't it. It's a simple fact, children get more attention in a smaller group.
I would suggest OP let DC go to this booster class, if DC really doesn't like it, then maybe give up, but do find out why he need this booster, is he behind? Maybe you can do some at home if he doesn't want to go.