I think it comes down to how much work you as the DM would like to put in. I think the idea of adding Mind-flayers here and there as other people have mentioned is great...IF you have the time to alter the encounters and so forth. Could add some nifty plot tie-ins with his character, could be pretty cool!
If you do NOT have the time or desire to add some illithids, let him know straight up that it will be a wasted effort, and you want him to play a character he will have fun playing through all levels. Point him towards something pertaining to Undead slaying. My God, if I could go back in time, knowing how much undead I would have encountered in 20+ levels, I would have totally changed my character build.
