Does he follow you when you click on the "regroup" icon (four arrows on the bottom left corner of the screen)?
If he doesn't then it most probably is some strange bug.
In that case we can try generic "solution to everything trick": are you using Vista or Windows 7?
If yes - then make sure that you install and run the game "as Administrator". This is just a guess, but it helps in so many cases that it's worth a try.
Edit: Stance shouldn't have any effect on coterie members following the selected character.
And on a side note: aggressive stance should be renamed to suicidal :P
Thanks for your suggestions guys, however I am not experiencing any fix. I've tried toggling everything: stance, pressing the regroup button, changing characters, the green "follow" buttons. I am also running this program as an admin since I was experiencing save game issues as well. Furthermore, Wilhem will not follow the group no matter who is leading; he won't follow Cristof, Serena, or Erik now that I have him.
In addition to being really annoyed that he won't follow, unless I am playing him directly, Wilhem will not fight during battle - he just stands there like a useless cretin.
I am running Win 7. . . *cry*