Hi, The first thing to review would be the parameters or restrictions of your nephew's visa. The U.S. Department of State website has information on tourist visas, as well as student visas:

http://www.state.gov/

Their website also has an entire section dedicated to youth and education: http://www.state.gov/youthandeducation/

Contact your local school district to find out their policies and procedures for summer school enrollment. If you don't have the district's contact information handy, you can research it in SchoolFinder:
http://www.education.com/schoolfinder/

Alternatively, you might also consider classes or workshops through your city's recreation department or local community groups. However, again, make sure that taking such classes or workshops would be permitted by the U.S. government under the terms of your nephew's visa.

I hope you have a wonderful summer with your nephew. Good for you for planning ahead in anticipation of his visit!