Your son is considered to be Self-Employed, which is great because he can not only report his cash income, but also claim a variety of expenses related to his profession.
For him, these could include supplies, cell phone, vehicle expenses and home office.
He needs to keep good records of his income and save receipts for his expenses.
Type 'schedule c' in the Search area, then click on 'Jump to schedule c'.
Click this link for a detailed article about Reporting Business Income and Expenses.