I'm with Jane...I had a cat, my daughters cat really, who had cancer in his face. The vet told me that as long as he was "ok" ie: eating, socializing, etc he would be fine, but once he started on the downslide then we'd have to make a decision. He stuck it out for awhile and my daughter was happy, but really, I let him go longer than I should have for my own selfish reasons. Cats do hide their pain well, and when it was apparent that he was dying, it was clear he was suffering and had been. I'm for putting him down now. Maybe have your son visit at the vets office, but make it clear to him that these things happen and the cat will be much happier when he's no longer in pain.
Sorry to hear about your cat by the way, it can be hard to lose a cat and it's really hard to make that final decision. But the cat will appreciate it, I know mine did.
SK