Feel free to penalize him in exp or rewards if he does not live up to this obligation. His alignment and actions should reflect that. Good or neutral mind flayers are practically non existent. If you use this final method, also force him to roleplay his character. It is usually better to use an idiot's stupidity against him than to force him to an unwelcome conclusion. See how he feels about outlandish builds after that. Allow everyone else to create characters normally at 18 or 19th if you want him to have a single class level.

Thus, his character would be the equivalent of an 18th lvl character even before his first class level. Together this adds up to LA +10 and 8d10 HD (the half-dragon template bumps the HD die up one). The half-dragon template (using the version in the MM) is LA +3. A mindflayer character is naturally LA +7 and has 8d8 HD. If neither of these work, simply announce you are using level adjustment rules without the buyout variant presented in Unearthed Arcana. No one likes being second fiddle in a roleplaying game, especially as there is an unspoken understanding that you are as important in a campaign as you make yourself. If that doesnt work, persuade the other players of how much of an advantage he would have. If he is as much of a rules-lawyer as you previously mentioned, it might dissuade him from the char, though it is strange how often rules lawyer DMs become munchkin players. First, try to tell him the build is illegal. While Im all for player creativity, this is too much.

D&d half dragon template