I can see both sides of the arguement and unfortunately it wouldn't be hard for him to sign up himself and do a bit of snooping in the public area. I don't know him, but I can't imagine it would be difficult for him to recognise some your posts.
The only thing I can suggest is to protect your private messages by frequently changing your password on a regular basis and ensuring you log out after each session.
Do you have SR on your phone? I find it great because it's descrete and you can access it anywhere, any time.
Also,
Have you actually sat down with him to show exactly what goes on here? (without being too specific, of course), explaining that it's 100% anonymous and that he shouldn't feel threatened for that reason?
Best of luck
RQ