Ai poor lovey, that does look sore. I don't think I'd send him to school like that. Possibly too vulnerable to knocks and scrapes leading to further infections, but also that if you don't know what causes the spots, you don't know whether he is infectious to other children.
Coxsackie IIRC is not a specific illness, but a group of viruses causing a variety of illnesses. I think they all cause fever and general poorliness, though. Did he start that way?
TBH (and I'm not a HCP) it reminds me of a relative who was immunosuppressed and was unable to fight off germs that others shrugged off. She kept breaking out in awful rashes and warty things. Which sort of fits with his history over the last few months?
Sorry, but it was over 10y ago, and I had very young dc, so I don't remember the details. She was exposed to my toddler when he began chicken pox, had a blood test the following day which showed that she was infected, and began a hefty course of Acyclovir immediately. It prevented the infection turning into active disease.
I think she had Acyclovir several times over the course of her condition, because virtually any virus caused her serious problems. Once her immune system issues had been dealt with, her skin cleared up. Now you wouldn't know she had ever been ill, but she has still to be careful regarding any risk of infection as she still has a weakened immune system.