Find out what is acceptable attire or if a uniform is required. The school likely issued a list of supplies/products that each kid is to bring the first day. Find out how the first week or so is conducted. There should be a good deal of communication from the teacher.
You should start prepping him now by talking about going to school but you can also start shopping for some school supplies like a backpack, lunchbox and anything else that you think you might need.