The information you are looking for is in Core Rulebook p555 under Ability Score Damage, Penalty, and Drain.
Relevant passage:
If the amount of ability damage you have taken equals or exceeds your ability score, you immediately fall unconscious until the damage is less than your ability score. The only exception to this is your Constitution score. If the damage to your Constitution is equal to or greater than your Constitution score, you die.
In short: yes he can go into negative ability score stats (effectively).
- Gauss
For every 2 points of damage you take to a single ability, apply a –1 penalty to skills and statistics listed with the relevant ability. If the amount of ability damage you have taken equals or exceeds your ability score, you immediately fall unconscious until the damage is less than your ability score.
It couldn't exceed it if it stopped at a point equal to your ability score.
It should also be noted that ability damage (unlike drain) doesn't actually reduce your Strength (to zero or otherwise). It gives you a penalty, which is only applied to strength-based skills, attacks, damage, and CMB. And, of course, makes you drop unconscious when the penalty is greater than your score. But it isn't actually reduced (so doesn't impact carrying capacity or feat pre-reqs or anything). It changed since 3.5, and is overlooked often enough to bear mention.
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Every poison attack I see deals damage to a stat.
Not quite - in the table of sample poisons in the PRD both Drow poison and Oil of Taggit simply knock you unconscious. And then there's Sassone Leaf residue: not only does it damage your con every minute, it also does 2d12HP damage each time (for up to 6 minutes).