The South Lake, over Bishop Pass, down to the JMT and return by way of Humphreys Basin and out at North Lake is a classic backpack trip of about 57 miles and takes you through absolutely gorgeous areas such as Dusy Basin, over Muir Pass with the Muir Hut, Evolution Valley then the Piute Pass trail through Humphreys. Many Boy Scout troops with boys just the age of your nephew find this to be a challenging trip but “doable”. Unless your nephew is a frequent backpacker he will probably find this trail moderately strenuous. It’s shorter than you mentioned but a gorgeous, on-trail hike. You could always continue on the JMT a little longer, then backtrack to the Piute Pass Trail for extra mileage.
When you end the trip at North Lake you will have to hitch a short distance back to South Lake for the car but it’s done all the time.
For latest snow/creeks info. call the pack station at Parcher’s Resort. Lots of info. on this trek from Trip Reports and online, in general.
With Bishop Pass/Dusy Basin or Kearsarge Pass be sure your nephew is acclimated to the altitude before you start. Some data reports say teenagers can have the worst reactions to altitude without acclimation.
Hope you have a great trip wherever you go!
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