I’m presuming they won’t let him bring the book home? I’ll bet that if you can wait a few weeks into the school year you’ll find a handful of used copies begin to hit the market, but I HIGHLY doubt that the 6th edition is substantively different from the 4th or 5th edition which you can probably get for pennies on the dollar. (Outside of David Christian’s Big History concept (https://school.bighistoryproject.com/bhplive), educational approach and new material hasn’t drastically changed world history textbooks in decades.) If you read through the linked article above, there are a few more ideas you might use to cut the price a bit.
For an AP class, you may be better off buying some supplementary popular non-fiction books (eg. Genghis Khan and the Making of the Modern World; Cleopatra: A Life; Guns, Germs & Steel; The Landmark Thucydides, anything written by Simon Winchester) which are likely to be more interesting, entertaining, and informative than the “assigned” textbook. They’ll help reinforce not only material, but reading skills, and critical thinking. You could also check to see if international editions exist yet and buy one of those at a steep discount (see the link in the article).
Digging around in free online MOOC’s can also reveal some excellent non-textbooks and references (like Jeremy Edelman’s History of the World Since 1300 https://www.coursera.org/course/wh1300). Try edX, Coursera, MITOCW, and the like for more possibilities.
You can also find some additional ideas via: http://boffosocko.com/2011/07/30/on-choosing-your-own-textbooks/