If the income was self-employment income and was more than $400, then he is required to file a return. While he will not pay "normal" income tax on that amount, he will pay self-employment tax on the amount over $400.
If the income was W-2 income then he is not required to file. However, if any taxes were withheld from that income then he may want to file in order to get those withheld taxes refunded. Understand that only the amount in box 2 of the W-2 will be refunded. Social Security and Medicare are never refunded.
