So let’s get these things straight:
Raspberry Pi is reachable on 2 computers that are connected to the same
WiFi network as the
Raspberry Pi. This means
dex.local or loading
dex.local page on the browser works.
When going over to another
PC (which has
Windows 7 on it) the
dex.local page isn’t loading and instead it returns a
DNS not found error.
To me, it looks like your son’s
PC isn’t on the same network.
Another possibility is that
dex.local has been blocked on your son’s computer - I’m wondering if
parental control software can do that.
It’s most likely that the computer isn’t on the same network as the other 2 and that’s why it isn’t working.
Also, on your son’s computer, could you type in
ping dex.local command and tell us what’s the output of it?