Tell him about school beforehand- casually- and talk about it as a happy place where there will be a big welcome for them  and where they will meet new friends
Don’t use school or the Teacher as a threat, even jokingly. “If you behave like that for Mrs. X she’ll murder you” though said lightheartedly can make some children very apprehensive.
If you feel it would help, you could take him/her for a stroll through the Infant Yards on an afternoon during June when the other children have gone home.
S/He will like to have their new uniform and new bag when they begin. These help him identify more readily with the school and with other children.