In my opinion, it depends on the amount of his time you take up. Remember that this guy works largely for tips, and that he is is lucky if he is paid for half the time he is at the airport, I would figure reasonable compensation would be at a rate of $20/hour for the time you use him. This would mean that he would realize $10/hour for the time he is on the job, a bit above minimum wage but not much. This means that if you use him for 15 minutes, you would tip him $5, if you use him for a half-hour you would tip him $10.
Just out of curiosity, why ask on the AA forum? Did you think that all disabled travelers would travel AA, or did you think that AA travelers would tip differently from those on other airlines? Did you know that we have an entire forum for
disability travel? You would likely get much better answers there.