As an exInfants teacher check with the school and ask if they have informed the parents of the child. Did your grandson tell the teacher he had been bitten? Tell him to tell everything that happens to him and check that the child has apologised and why he was bitten in the first place.
Take it slow and calm but get all the facts from your grandson before you approach the school though as it is Friday I think I would mention it to them and find out if they knew about it and how they had dealt with it.
Shall we reboot our cartoons thread again? 😁