According to EW's Ausiello we will meet Jack's grandfather for the first time in Episode 5x07 316 and he will be played by actor Raymond J. Barry.
Based on a previous casting call there was character in his 70s by the name of Hal who supposedly "an still fight the establishment and win". So is Hal Jack's grandfather??
No other details are known, but here is a link for Barry's IMDB profile.
Raymond J. Barry on IMDB