Backup all those identities folders before you move them around, just incase you over-write something by accident because some of those dbx files could all have the same file name.
I would do what gokudx7 said, I've done it before except when you locate the dbx files with your bosses emails on them, copy them all to whatever folder your outlook express is currently using (over-writing some like inbox if it has to) and restart outlook express. Everything should open up like normal with all his emails next time he opens the program.
Alternate:
Heres some links to programs that restore outlook express dbx files. As long as those dbx files are on the computer you could use these to restore them (these could be useful if things are really bad like corrupted files.) This might be the faster route if you can't distinguish what dbx files are what.
http://www.mailnavigator.com/restore_outlook_express_dbx_files.html
http://www.newfreedownloads.com/find/dbx-files.html
I've never used these programs before so I don't know how effective they are.