logging sftp transfers

I have a directory with files I want my son to have access to. I have set him up an sftp account, and made him a member of agroup with limited privileges. But I would like a list of what he has downloaded -- he lives in a shared house, and I don't regard my computer as a shared resource. He might perfectly well leave a program running ... Is there any way to log the files that are transferred by the sftp server? I have googled at length, and in vain. -- Andrew Brown What I do: www.darwinwars.com What I'm up to: www.thewormbook.com/helmintholog/