Grandfather can be used as a formal term, even a term of respect for an older person to whom you are not even related. it would be easier to call him Grandfather than Grammpa or whatever, but it may be easier for you to call him Mister Whatever-his-name.

I understand your wariness, and think it is okay. This is a chance, though, to reconnect with that part of the family both for your own sake and his. I would give him a chance.

Isn't email great! It gives you more of a chance to think than a phone call...