OK that is a lot of information, I’m gonna try to break it down.
1.So you are in the SAME network, then you should have no problem with the direct connection part. If you both connect to the SAME router, you most likely have IP addresses in the same range (e.g. you have 192.168.1.110, your son has 192.168.1.111) and should be able to connect to each other without problem. The TURN server would not be involved in your case. You can toggle it off in the “account” tab, “advanced” option page, if you are still able to connect, then you can confirm that you and your son are indeed directly communicating with each other over local network. Upnp is not required neither.
I thought you were at different locations, because normally calls would not happen for people in the same household. You only need to worry about connectivity issue when you are using different networks, i.e. home/work, home/mobile, mobile/mobile(most tricky type) etc.
2.It seems that your main problem is with audio devices, that is very complicated, because maybe it’s your motherboard, maybe it’s your headset, maybe you system lacks some driver, could also simply be a bug from software, there are a lot of possibilities, my only suggestion would be to switch devices, use a different PC, a different phone, see what works and what does not. Can’t really help with it.