Shalom jacquiw
*Jewishness* is determined by Ultra Orthodoxy through the maternal blood line NOT the patriarchal.
Since your grandfather's proclaimation to follow chritianity it is MOSt probabble that the entire family was dismissed from the linage to be considered should grandfathers name arise.
It may be possible to trace the family line through the mother of the grandfather. Still, the branch of Judaism MOSTlikely to embrace your quest would be REFORM.
Baruch HaShem!
be fitly blessed,
aTeeDAZ