Why does Matt's father leave him alone in the wilderness in Sign of the Beaver?

Matt and his father had traveled to their holding in Maine territory to build a house and get some crops planted before bringing the rest of the family over.  Matt's mother is pregnant, and for her comfort and safety, she and Matt's younger sister stayed behind at their home in Massachusetts.  The plan is that when Matt and his father have prepared their holding so that the basic amenities of shelter and food would be available to them, Matt's father would journey back to Massachusetts to fetch the rest of the family and bring them to Maine.  It is calculated that the trip will take six or seven weeks to complete, and during that time, Matt will stay at the holding to care for the crops and protect what he and his father have built.

Matt is only twelve, going on thirteen, but he has been raised, like most children at that time, to be able to take on a great deal of responsibility.  He had "helped to build every inch" of the log house in which the family will live in Maine, and he is quite capable of fending for himself and taking care of the corn which the two have planted.  Still, it is a very large undertaking for a young boy to stay alone in the wilderness for so long a time.  Matt is proud but a little afraid, and his father is a bit reluctant, but knows there is no alternative.  Matt's father is secure in the knowledge that he has trained his son well, and has faith that Matt has the maturity and strength of character to follow through.  As a sign of the confidence he has in his son's ability to do this hard thing, Matt's father gives him his most valuable possession, an old watch which had been handed down to him from his own father.  He also leaves the boy another valuable item, his good rifle (Chapter 1).