Here's my perspective on wishes in general. I don't use ELH (too over the top for my taste), so IMC a wish is the most powerful magic a mortal can hope to weild. The force of the magic will do what it can to make the character's desires come true. The 'safe' wishes in the PHB are examples of what is within this range of power. Anything beyond that may have unintended consequences. IOW - wish for too much and you might get screwed.
For instance, I once had characters wish that 'Last month, I'd been returned to life by True Resurrection rather than Raise Dead.' Seeing as how this is one of the fundamental balances of the game (and I did warn them) it was beyond the power of the wish. But I do allow wishes to rearrange the world in order to make the wisher's desire come true.
So time unravelled, and a high level cleric of Nerull that had been plotting evil schemes in the distant mountains was transported to just outside town. And the characters remember agreeing, via speak with dead' to provide a service under magical compulsion in exchange for a True Res. Time wrapped back up, and the characters were a level higher. But now they had an evil high level cleric in their midst and a quest they HAD to perform for him!
If your case, I'd probably allow it. A feat is not an unreasonable thing to wish for, even if he doesn't meet the prerequisite ability scores. I'd be more cautious if it were at the top of a feat chain or something. If you thought this was unreasonable, the wish would find a way to make it true. Unravel time, the character takes a sabbatical of intense conditioning of his reflexes, gaining a +3 bonus. Unfortunatly, he missed part of the groups adventures, so dock him a level. Take the last feat he chose, and change it to dodge.
Like I said, I wouldn't stick it to him like that for what he's asking. I always warn my players if they are pushing it too far. SOmetimes (such as with the Res) they decide its worth the risk.