Better to let him stay home for the time being as I believe the school will need to do cleaning and disinfection. You can wait and see if there are anymore new cases as well. If it is a sole case then it's not so bad. However, do take note that Hfmd can be contagious during the incubation period even before symptoms develop so if your LO's immunity is low, he might get the virus. Try not to let him touch the newborn baby for now.
I believe school will provide guidelines to parents follow the guide, if you worry let him stay home