Best just have the chat than let it play on your mind. It's no big deal. Little curious though how you would take it if it were him wanking unknowingly in his sleep then woke up to see you with a phone facing him. Would you beleive he was genuinely sleeping and would he beleive you hadn't fixed it? Either way, it takes 2 minutes to clear the air.
First, find out what kind of phone he has because some cell phones' pictures are less than desirable. Then turn him on to this site and have him download the videos and Pics here under the appropriate thread title and wait for the compliments to come rolling in.