Unmarked spoilers are ahead, per Spoilers Off rule.

Tropes for the film:

Charles confesses to Will that he was unable to save Will from drowning many years ago; Jim's father was the one that pulled Will up out of the water. When Charles says, "You have to stop blaming, sometimes," he seems to be trying to tell himself that as much as Will. Will's response: "I wish you could be happy." One suspects Will figured out long ago that his father wasn't able to help him, and he feels no ill will whatsoever. Charles doesn't realize it — yet — but Will accepts his father's flaws and regrets, and loves him no less for them.
In the film, Will is able to break Mr Dark's thrall over his father by calling out "I love you!" (no easy feat for a preteen boy). Jason Robarts's face lights up with a hopeful smile, and this compels Charles to smash through the mirror and embrace his son.