Mr. Jacket is a guy that Sam meets twice in the book. The first time is when Sam goes into town. He meets Mr. Jacket at the drug store. They talk some because Jacket is intrigued by Sam's clothes. Jacket is said to be about the same age as Sam.
Then later, almost at the end of the book, Mr. Jacket comes out and finds Sam out in the woods. The two of them catch the big snapping turtle. Then they go up to Sam's house in the tree. We find out that Jacket's name is Tom Sidler.