If the rest of his internet connection is working fine, that is unlikely to be the problem.
Check your PS3's internet settings, what firewall type does it say it is? Is it NAT1 or NAT2? (if it's NAT3, you will have issues). Check whether it says UPNP is available. If not, try updating the firmware on your router. If that is too hard, try checking your router's Port Forwarding and enabling the ports that GT5 requires (google "PS3 GT5 TCP/UDP ports").
Failing that, is your disc scratched/dirty? Clean it and give it another go.