as he is getting downloads of somebody else's photos and has had to disable his own photo stream.
How does he go about fixing this problem? Is it possible to cancel his existing iCloud account and starting a new one?
Change the password.
Changing the password is absolutely the first step if you suspect the account has been compromised, but are you certain he hasn't just accepted the invitation to a Shared Photo Stream. In that case he would get downloads of photos from anyone who added photos to the Photo Stream.
You can read more about Photostream features here:
Photo Stream Features: http://www.apple.com/icloud/features/photo-stream.html
Using and troubleshooting Shared Photo Streams: http://support.apple.com/kb/TS4379
Photo Stream FAQ: http://support.apple.com/kb/HT4486
My son suspects one of his friends is using his iCloud account
