Strictly speaking, it's none of Cameron's business. All lot of people would have liked to tell Cameron the same even years ago.
Jeremy Corbyn has had a lot of grass roots labour support. He is unfortunate that he inherited a lot of Labour MP's that were not proposed through the current party system, but were list appointees. Many are Blairite right wingers/apparatchiks and have just been waiting for an excuse to politically assassinate Corbyn. Personally, I think Corbyn has many good qualities, even if I'm at severe odds with some of his policies. On balance he comes across as a decent man, with generally good intentions. I have a lot of sympathy for his current situation.
http://www.reuters.com/article/us-britain-eu-corbyn-cameron-idUSKCN0ZF1HA