Yes I think you and the nurse should change your son's diaper. I wasn't able to change my own diapers until I was 2 years older than him. When I was in school the nurse frequently changed my diapers. When I was 6-7 occasionally my teacher would change my diaper and I was teased quite a bit for it because everyone knew I wore diapers. Having the nurse do it is a little less likely to be noticed so long as he manages to have his diaper changed during recess and at lunch. If he needs his diaper changed in the middle of class, chances are that eventually the other kids would figure it out.