Well, don't forget a belt of magnificence +6. A nice fat bonus to every ability score for a mere 100k. Stack on a monk's belt for Wis and an additional +1 to AC. Assuming you don't want armor, anyway. Getting a higher bonus (ie, epic belt of magnificence) is even better.
The belt also frees up your neck, forehead, hands, and shoulders slot for better items (for Con/Wis, Int, Str/Dex, and Cha, respectively).
Also, aging will increase your mental stats, and since you're polymorphed all the time, you don't have to worry about the penalties, so you can put more points into your mental stats without worrying about penalties to your physical stats (unless your DM uses the 'no hp changes via polymorph despite changes in Con' crap).
You can also get additional bonuses to Con by creating items that grant other stackable bonuses, though they'll be very expensive.
Also, if he plays with psionics is completely different, a psionic bonus to con (animal affinity) will stack with magic bonuses (though this is cheap and the 'psionics is different' variant is utterly broken; NOT a good idea).
There's also the trick that Pun Pun uses to increases his ability scores - bellflower tattoo/voidshaper, etc.