Maybe you could subtly ask his office staff what form he might prefer. Otherwise I'd probably not abbreviate it.
Poppa and Gran, Pop and Nana or Nan. Gramps and Grannie. Pa and Ma. Pa and Nan. The possibilities are endless.
Don't think I've ever seen it abbreviated!
I want to thank everyone for their input I tried full grandpa and the G'pa and I had a meeting with him yesterday he decided he just wanted the year. So again thank you. Every one is always so helpful. VLP Now off to make the Crampons' Bags for grandchildren. Mountain design with baby foot print on one side little tiger on the other will share when finished.