I try to report his post, but he locks it and I can't find the option to report it.
Are locked threads not reportable?
They can still be reported. Just look for any locked thread and look at the bottom right just below the post, you will be able to see the "Report to Moderator" option. I think OP was not keen enough to see the report option.
There's no reason a report option would be disabled, otherwise we would have malicious folks doing that every other time here.