Is it a proper shirt or polo shirt he has to wear? Keep the buttons open. Wash a few times to make softer or buy some secondhand which are already quite soft. If it's a proper shirt that is more difficult but hopefully seeing other children will help.
Maybe get some choices eg. 2 shorts, 2 trousers, plain white T shirt with round neck as well as some normal shirts. Choice of socks and pants. That way he can pick what to wear for first day and everyday.
When T shirts get dirty and go in wash he won't have them to choose from. He will understand by then what uniform is. He will gradually get used to the collar ones if he can sometimes wear the normal one. Only do washing once a week so he knows if it's dirty then it's dirty.
Don't make a big deal but you can do 'brain washing' subtly such as watching programmes with school kids like a PP suggested, talking about what rules nursery has to remind him there are rules in school (without mentioning uniform!), asking him if he can remember what colours his school uniform is, finding pictures of other uniforms like doctors, firemen, police and talk about what they're wearing.
If he chooses the white T shirt on his first day, that's OK. I think he will get used to it when he sees other kids. Just be firm that when it's dirty it is in the washing and will get washed at the weekend so he needs to choose a different white shirt.
Does he have any other sensory issues or just shirts? If it's just the shirts then maybe he's just digging his heels in. If he hates certain other clothes then it may be sensory.