Hello J. ~
I've only known of an infant having corrective surgery for hypospadia & that went well. In some cultures, circumcision is performed at age 13, so I suppose this wouldn't be unheard of. I'm presuming that surgery was deemed unnecessary when your grandson was an infant & now he's wanting it done because he's wishing to be like other boys? I hope it's that and not that there have been complications arising.
In any case, it would be for a doctor to advise you, as the other moms who've commented on your post have already said. But I have a feeling you were hoping to find others with similar experience. I don't know how common it is for a 14 year old to have the surgery, but you might want to re-post your question stating that you are seeking to hear about how it went for anyone out there whose son or grandson had the surgery as a teenager rather than as a baby.
Also, have you tried googling your question? Apparently, it is rare for the surgery to be done later, but I did find a website with information & advice. This might be a little bit more information than you want right now, because it's a Q&A from men & teenagers, so some of the concerns were whether they'd be able to perform sexually, but do give it a chance, scroll through to find questions that relate to what you & your grandson's parents & your grandson need to know:
http://heainfo.org/FAQ-PeopleWith.html
I'm sorry he's having to deal with this & I hope the right course of action will be clear very soon. I wish you & your family all the best!