Give him a Spectator(LoM) instead: if he refuses, that means he's aiming for a Beholder Mage, just tell him that it's forbidden; if he accepts, then it's fine, just put him at ECL 6(+2 LA) I think, maybe even 5(+1 LA), as they don't give beholderkin's LA, then you just count him as a lvl 6 or 5 for distributing XP, the players will eventually catch up and then he can just advance as some class.