That is indeed baffling.
Just to emphasize in case you missed it, that missing this would cause it.
You might also try, just for good measure
sudo chmod 775 /home/wine -R
But to be quite honest, that seems redundant and it really sounds like you've followed things properly already, unless you missed the 'chmod' commands.
sudo chmod 775 /home/wine
sudo chmod 775 /home/hunter/.wine -R
sudo chown :wineshared /home/hunter/.wine -R
sudo chown :wineshared /home/wine -R
Maybe, and I'm pulling at straws, if you remove the symlink from your son's account,
and then log in as him and create the link again using his account. So from a terminal at his home directory.
sudo rm /home/hunter/.wine
If it won't run like that, the last thing I can think of before surrendering to being stumped, is telling wine where the wine folder is, instead of using the link to the folder, so he would start an application by, at the command line, instead of doing something like this
ln -s /home/wine .wine
try like this
and if that works, athough I think it won't, then you can actually get rid of that WINPREFIX part by exporting it as his own variable on login but that's getting way ahead of where we are. I'll keep the page bookmarked in case it does work in that manner but I'm not very optimistic, and quite frankly I don't know why it didn't work so I'm really sorry I couldn't be of more help. Hopefully more people look at the thread despite it having multiple replies already. I'm stumped.
WINEPREFIX="/home/wine" wine "/home/hunter/.wine/path/to/some/application.exe"