How does Grim demonstrate that he does not trust his grandson initially but then slowly change his mind in Freak the Mighty?

In the beginning of the novel, Max's grandfather, Grim, remarks to his wife, whom Max calls Gram, that Max looks so much "like Him," meaning Maxwell's father. Maxwell notes that his grandfather will never speak his father's name; it is as though it "is too scary to say." Max narrates,

Grim says that night in the kitchen, "...the boy is like him, we'd better watch out: You never know what he might do while we're sleeping. Like--his father did." (Ch. 1)

Thus, Grim tells his wife that it is more than Max's physical resemblance to his father that bothers him; it is also Max's behavior that is similar to his father's. Apparently, he is very worried that Max will turn out like his father.

After Max meets the small boy who is called Freak, they become friends. Coincidentally, Freak's mother, whom he calls Gwen, was once friends with Max's mother. She knew his father, but Gwen will only tell Max that his father made it difficult for his mother to have friends. Later, in Chapter Seven, Freak and Max watch the Fourth of July fireworks; unfortunately, however, while they walk around, they encounter Tony D, a gang leader. Freak boldly calls "Blade," as Tony is known, a "cretin." But as luck would have it, a police car passes them, and Blade runs off.

In Chapter Eight, Max's grandfather gives Max a "pruney" look when he sees Max wet from the pond after he is brought home by the police from where he and Freak watched the fireworks. While they turn over Max to his grandparents, the police (who have given the two friends a ride home) tell Max's grandfather that Max rescued "the poor crippled midget kid."

So Grim listens to the cops and then he gives me this weird look, like, imagine my surprise....(Ch.8)

When Max is dry, his grandmother offers Max some ice cream, but his grandfather interrupts, saying that what Max should have is a cup of coffee. "Real coffee," he adds. After Gram has made this coffee, Grim hands Max the cup. Max narrates,

Grim is handing me coffee in a china cup, from the set they never use. He gives me that cup like it's a really big deal, maybe because I'm not allowed to drink coffee yet, and he's so Grim-like and serious I open my mouth to say what's the big deal...and something happens. (Ch. 8)

Max realizes now that his grandfather is proud of him, and he recognizes that Max is not like his father. Grim beams at Max with pride.