Is the maternal grandfather considered like the father, and is it permissible for him to address his grandson as "my son" or "my boy," and is it permissible for the grandson to address him in a similar manner?

The point of resemblance between the father and the grandfather is not clear. If the intention is resemblance in terms of mahramiyya (prohibition of marriage), then the mahram relatives by lineage are the fathers, however high (from the mother's or father's side), and the sons, grandsons, and granddaughters, however low. So, the maternal grandfather is considered among the fathers, and his grandchildren are considered among his children.