When it gets closer (a few days before) start talking about what might be expected
ie, they will sit on the carpet and take the register so that they know who is in the class on the day.
They will read books, listen to stories, paint, draw, might play in sand, play with toys, build things from anything and everything.
they will start to learn about letters and numbers, counting, etc.
There will be times they have to sit quietly and listen to the teacher and not talk
They will have a routine, breaks, lunch, etc. times to drink water, eat food.
Emphasise its fun, makes new friends and he will enjoy it.
Most importantly is ensuring your child can put clothes on and off, and quickly. Hang a coat up. Put a coat/hat/gloves on and fasten coats.
Ensure shoes are easy to take on and off.
Know how to open lunchboxes and their contents etc.
These are the things that can cause more stress than the actual day.