As long as the 14 yr. old wants to be there, treat him with respect and give him lots of stuff to do.  Tell him it's a job you usually reserve for an adult, but that you think he's mature enough, strong enough, etc. and up to the task.  Give him a lift and let him soar.  Also, ask him what he wants to do, and within reason, let him do it.  Make games out of stuff.  Let's see how fast can we get this portage done.  Guess how long until the sun sets.  Think we can beat that other canoe to the island.  That kind of stuff.  Make sure he brings a book, or get him a book that has some significance to the area.