What's the problem here?
By 11th level, a wizard could make a headband of Intellect +6 (for 18k gp and 1440 xp) -- less with "magical artisan" feat, so he's not at all out of line for an 11th level character. (When I play a wizard, I save my coppers until I can make this item, and end up with it fairly early -- like 8 or 9...) A human that started with an 18 Intelligence and pumped intelligence at levels 4 and 8 could make this headband, and have saving throw DCs 1 better than the pimped out Sun elf, before spell focuses. I don't see the problem.
Wizards, especially invokers, are very good at crowd control -- that's their job. He burned 2 feats to do it! He should be good at it!!! The peons get dropped by the fireballs while the heavy fighters go in to kill the BBEG. That's the way it works. Lord help you if he choose Enchantment instead!
So, what's with the other folks in the party? He's made his guy pretty effective, why haven't they? Is it ability score differences? Then even it out... (We use a system where the score modifiers (at level 1) add up to +11 or +7 (depending on the flavor of the game), with half of the scores odd, so everyone starts on an even keel.) Are you throwing hordes of crowds at them? Then throw in a BBEG or two. Split the crowds into groups of 2 or 3, and have them come from all points of the compass, so very few can be hit by a single fireball.
What I'm really saying is that this guy might seem more effective than the other because:
1. Luck of die rolls in PC creation; (than even it out)
2. He's constantly in situations that maximize his skills (use other situations), or;
3. He's a better player than the others (get them in gear!).
Of course, a 6 strength is crippling -- he NEEDS a Heward's Handy Haversack just to carry his spellbooks.
OfficeRonin