Was the kind whiny and made bad choices? Sure. But they were the kind of bad choices and whininess that made sense for a kid his age. I remember making similar bad choices that didn't involve griffins. What I liked about the story in large part is that it felt pretty true to how something like this might go down, rather than sugar-coating with nostalgia.
I did think it was a little weird that he was so afraid of them hurting the griffin and not the other way around, but I don't think it's implausible. Throw a pack of teen/preteen boys trying to show machismo for each other, with enough forewarning for them to grab air rifles and firecrackers, put them together with a mythical wild animal out of sight of adults, and I don't think it's implausible that they'll cause some damage. They could shoot the griffin in the face with an air rifle, or break one of its legs or a wing, or hit it with a bottlerocket, that kind of thing. The most likely RESULT of that will be that either the griffin will run in startlement, or it will tear some guts out. I don't think it's too likely the griffin will be killed, but it could be injured. Whether it's injured or scared away by firecrackers or something, it's probably not going to come back, which would be a major loss for the protagonist. And if kids die in his backyard, probably witnessed by other kids, even though adults won't believe it's a griffin, there's still going to be a lot of hard questions to answer, and a lot of adults poking around those woods, which will probably mean the griffin won't come back.
I did think the ending was the weak point of the whole thing, the rest of the story was very engaging for me, but it kind of ended with an ellipsis...