Request #584 From Wren

I’m searching for a fantasy children’s book that I read over and over as a kid. Sadly I don’t remember too much, but I do remember the main character was a boy and he had a small box(small enough to hold with one hand, possibly a gift from his mother?) and he was warned to never open it. It gets stolen in the beginning and the boy has to go on a quest to get it back before it gets opened. I don’t remember too much about the cover, but I think the boy has was riding a giant bird of some sort? Please, if anyone knows what I’m talking about I would be so grateful to hear.