Check what ports LW's set to in your son's account....Open LW and go to Tools>Options>Advanced>Firewall Config, and see what the listen on port is. You'll need to allow exceptions for that port through your firewall.
When you have LW running on more than one account, you have to use different ports for each one. Or set his port to the same as yours but you can't have both running at the same time. If you do, the one that's opened second will pick a different port and will not be able to connect.
My best guess, hope it helps
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