Sorry, he won't owe any regular income tax on it but will owe self employment tax on $400 or more. If you file Schedule C for yourself you probably know how it works. He files the same way on his Schedule C.
You pay Self Employment tax (Scheduled SE) on a Net Profit of $400 or more on Schedule C. You pay 15.3% SE tax on 92.35% of your Net Profit (If it is greater than $400). The 15.3% self employed SE Tax is to pay both the employer part and employee part of Social Security and Medicare. So you get social security credit for it when you retire.
