Hmmm, for a kid his age I would say either a .410 shotgun or a .22 Try taking him out in the early morning and evening. Squirrels seem to move around mostly when it is cool. You could buy him a squirrel call, they get suprising results.

Good Luck This Season: Buck Magnet
P.S. Only Chuck Adams can sit at home and see deer so get into the woods. =;^)