You may want to request a birthday greeting from the White House for your grandfather. For my grandmother's 88th birthday a few years back I requested the birthday greeting. She was totally thrilled when she opened the envelope!!
go to: www.whitehouse.gov/greeting/
for specific directions in making the written request.
It takes 6 weeks to fulfill the request, so act quickly.
Whatever your plans will be for celebrating your grandfather's 100th birthday, I hope he enjoys the day and the company of his family and friends!