Forwarding his post to him must seem like a very good idea in retrospect.
I believe all he can do is appear in response to the summons and either plead his case or have a solicitor appear to plead on his behalf, which is what I'd strongly suggest. After that it's up to the judge.
Is his motor insurance also registered to the wrong address, i.e. the one where he doesn't live? Is he insured in his own name as the main driver or on someone else's insurance as a named driver? If either is the case he needs to look at regularising the situation. I'm not criticising, just expressing an opinion.