I have never registered to vote and not had any problems yet. In your case just say he doesn't live there any more? Maybe I get away with it because I live alone or its just council specific.
I still struggle to understand why it is mandatory to register to vote even if you are not voting.
A word of caution.
The law under which electoral registration is run changed recently. The old system of household registration where one person was designated responsible for registering everyone at the house has gone, and is replaced by individual responsibility where each and every one of us, individually, is required to register unless you fit one of the statutory exemptions.
Until recently, by which I mean late 2013 if memory serves, you could be fined, and up to £1000, but it required prosecution, a court case and gave you a criminal record. So, needless to say, there weren't many prosecutions .... if any.
However, now, you can be issued what amounts to a fixed penalty notice, like parking ticket. It is much, much less onerous a process on the courts, and my bet is that while going through a whole prosecution process was a big deterrent to the authorities, they will be far more proactive about handing out £80 tickets.
My point is, just because they didn't bother chasing in the past doesn't mean, under the new far easier system, that they won't in the future.
As for those recommending the OP fill in and sign the registration for his brother, my advice would be to look up the penalties for electoral fraud before doing that. The declaration is a legal form and if they catch you fraudulently completing someone else's form ....
A better bet would be to contact the council's electoral officer, explain the situation and drop it in their lap.
