Do you have a nurse line you could call?
Looking it up, I see "Infrequently, the infection may result in temporary fingernail or toenail loss". Since there's no actual treatment for coxsackie, I imagine there's no treatment for this, either, except for patience.
I've had two nails off before, both due to trauma, and while it was WEIRD to feel the breeze on a part of me that is normally covered, there was no *problem* with it.
I personally would just call the nurse line, see if they have any advice on helping him with any weird sensations, but honestly, personally, I doubt I'd even do that. It's not an infection; it's the aftermath of one.