Grandpa Arnold calls his son John when he visits, but everyone else in the show calls him Jack.
The young Kevin in the Christmas flashback is Eric Lloyd, who went on to play Charlie in another Christmas movie, The Santa Clause.
-CONTAINS SPOILERS-
While with his dad and the dog at the end, Kevin said his dad would one day be the grandfather of his sons. But Jack wasn't around when Kevin's children were born, as stated at the very end of the series.
While with his dad and the dog at the end, Kevin said his dad would one day be the grandfather of his sons. But Jack wasn't around when Kevin's children were born, as stated at the very end of the series.