The company does not hire him but just pays him for these jobs.
So at the end of the year the company prepares a document for him called a 1099 form and sends it to him and files it with the IRS in their company tax filing.
It has his information with Social Security number etc. and with the amount that he earned for the year.
The W9 is easy to fill out
for him because it asks for name, address, social security number or TIN (Tax Identification Number) whether he is a sole proprietor etc. and date.
For information about filling out a W9 look to pages 2, 3, 4 for the instructions.
Fill out the 1st page and submit it to the payer.