Hello PURELUCK1999,
I am sorry to hear you are having this problem. In my experience, having two Playstation's behind the same router typically requires a bit of network troubleshooting on both consoles.
You were absolutely correct in assigning a static IP to your Playstation 3 and setting that as the DMZ host. I usually recommend when running two Playstation's on the same network, that you assign both consoles separate static IP addresses, assign one as DMZ host. Typically the one that is used the most should be set as DMZ host of the router as it ensures that no ports are being blocked and you have a completely open connection, exlcuding built in firewall/security features of the router. For the other console, you would then forward the ports.
The reason I recommend this, is because when you have two Playstations behind the same router/home network, this can often cause server issues where it will identify both consoles as one (based on the way the IP shows up server side), and cause those kinds of kicks.
Once you assign a static IP to your son's console, try forwarding the following ports for that IP on your router;
80, 443, 3074, 3478, 3479, 3658, and 5223.
Some of these ports are standard Playstation Network ports, and may improve your connection in other games as well (3658 is the standard port for voice data transmission on Playstation 3).
