That Guy (1999)

Yuta, an absent-minded 5th-grader, is sent to spend the summer with his grandfather, whom Yuta's parents think is becoming senile and could benefit from Yuta's companionship. At first, Yuta thinks Gramps is a bit addled, too, but as he gets to know the old man better, he finds that his grandfather sees things quite differently from other people. The special bond that often develops between grandfather and grandson grows stronger and stronger until the magical and heartwarming finale.