We could go with a unique progression. He originally had 5/5/5/5/5/3/3/2/1, which I think would work. That's only 1 time stop per day, and only 2 8th-level spells.
Attempted revision
Spells: Luthax may expend stored magic levels to cast his known spells. Luthax casts spells as a sorcerer in most respects, except in addition to using spell slots, he must expend a number of stored magic levels equal to the level of the spell. For example, Luthax must expend three stored spell levels (in addition to the normal 3rd-level spell slot) to cast haste. As a side effect of his transformation into a ghazneth, Luthax has a unique number of spells per day, and does not gain bonus spells based on his Charisma modifier.
Sorcerer Spells Known (5/5/5/5/5/3/3/2/1): 0th--light; 1st--magic missile, sleep; 2nd--blindness/deafness, invisibility, knock, locate object; 3rd--blink, deeper darkness, haste, hold person, invisibility sphere, wind wall; 4th--charm monster, confusion, dimension door, solid fog, stone shape; 5th--passwall, telekinesis, teleport, transmute rock to mud; 6th--control water; 7th--mass invisibility, phase door; 8th--binding; 9th--time stop. Caster level xth. The save DCs are Charisma-based.