The business code is for statistically purposes; you simply choose the number that best fits the description of what he does. You'll find the codes at the bottom of this IRS resource by category: https://www.irs.gov/instructions/i1040sc/ch02.html
You can also just enter 999999 which is the miscellaneous code. Anyone who starts working for themselves without forming a separate business, is considered to be self-employed, ie. have a business. Many people receive a form 1099-misc with nonemployee compensation, which means, for tax purposes you have your own self-employed business.
