Actually, he has a better case on Cal and Az than on Texas. Texas became independent following an internal revolt of its own citizens. That has long been recognized as a legititate means of establishing a new government/nation.
Cal & Az (Nevada, Colorado, New Mexico also, I think) we TOOK from Mexico after Winfield Scott handed Santa Anna his hindquarters on a platter. Since the Mexican War was fought over the enlightened cause of manifest destiny, Mexico might just find some court willing to hear their complaint.
That court might even call for a referendum.
I don't to may Scotsdaleites or LoxAngleirons are gonna vote to be wrapped in the warm loving embrace of stout, but poverty-stricken, Mother Mexica.
California was declared an independent nation following the Bear Flag Revolt and then annexed by the USA.