Late 90s(?) book about a boy who climbs trees

I'm looking for a book I read back in 1997/1998/1999 (I think it was released one of those years).

It's a young adult book about a boy who was born deformed but he was very strong.

He loved climbing trees in his backyard and there's a phrase in the book that goes something like "he didn't know where his backyard ended and the woods began".

He also spots a 'laughing girl' from his house in the distance, she is accompanied by an old woman who the boy later finds out is her grandmother.

There's also a plot detail about how the boy keeps certain items precious to him in a drawer, however through his adventures he finds actual valuable items including a red ruby(?) and it's said in the book those replaced his existing collection as they were much more valuable.

I think the books cover is greenish and taken from the perspective of the protagonist looking down on the ground from a tree he's on.