LRavenscroft
Does he go to a private/public school? Teachers need to make allowances for children in all sets of circumstances as in the case of your grandson off ill. If it were my child and he was given a detention for being off ill, I would be straight to the class teacher and head. However, in some private/public schools they may come back with the argument 'Is this the right school for your child?' Basically, you need a very strong adult behind him so he feels secure. I speak from experience. Good luck!
But there's no suggestion on the attachment that he's to be given a detention.
This is very different from an end-of-year exam or GCSE, which try to differentiate between candidates by having a spread of results. The idea with these regular self-quizzes is to make sure all pupils grasp the basics before moving on. Pupils are given Knowledge Organisers with a summary of the basics, so that they have all the information they need readily accessible, even if they've been absent.