I'm familiar with prolonged QT intervals. In short, yes, it's something to be aware of and concerned about. NO, it is not something to be scared to death about the prognosis. Make sure that your grandson has a good pediatric cardiologist, follow his/her advice, and everything should be fine.
It is treated, at times, by medications. Othertimes, they will implant something similar to a pacemaker called an implantable cardioverter-defibrillator. Either way, they can and will take care of him.
So, take a deep breath (make it 2 or 3), and take it easy. Things should be just fine.
Any questions, just ask. Enjoy the Magic of WDW!