As it says he still has 450 posts, but only posts/topics as his current username appear and searching doesn't seem to find much
can you PM us a link to his user profile? sounds like a bug, but as @baris says, all posts are tied to UID so changing username won't make a diff, but possible something else (deleting posts?) could affect that. Tried fooling around a bit on my dev env, couldn't repro so I don't really know.