As many accounts of what's happening from the people it's most affecting, I think. Anyone who has loved ones in Azkaban, parents who have spoken with their Hogwarts-aged children recently. People who have had their distress calls denied. In fact, if anyone has a suggestion on how to compile those accounts (audio recordings? something?) so that they're hard to ignore, I'd appreciate it. Letters don't convey quite as much, I think.
The most important approach, I think, is to keep reminding him that YKW isn't going to stop with the UK.