While I understand your concern for your son, why should the whole world be penalized when your son could just stay home in a safer environment?
There is no right answer to this as someone will be deprived of something somewhere along the line. But it seems to me that if it’s a choice of one person staying home or 33 people staying home, the one should stay home. He didn’t do anything wrong, they didn’t do anything wrong, but someone has to stay home. May as well make the damages as minimal as possible.
Because it isn't that simple. If it was just a matter of my son missing 21 days of school (the incubation period) that might be entirely reasonable - but the problem is, once you have an outbreak that can become a rolling number.
Kid A gets chickenpox and infects Kid B, who 21 days later gets chickenpox and infects Kid C, who 21 days later gets chickenpox, and infects Kid D, who 21 days later gets chickenpox and infects Kid E, who 21 days later gets chickenpox and infects Kid F...
What you end up with a long term outbreak. The only way to stop that happening if you have a population with a significant number of non immunised kids is to take all of those kids out of the school for the initial incubation period. That includes kids like my son. Once the school has been chickenpox free for 21 days, and none of the kids kept out of school have got it, and you know none of the unimmunised kids have been exposed, you can let them all back in.
Keeping the immunocompromised kid at home won't stop the outbreak. If you've got 33 unimmunised kids in the school, the chances are quite high of a rolling outbreak.