If he receives a W-2, the amount he can contribute to a Roth IRA is the amount shown in box 1 of the W-2.
If he is an independent contractor and not an employee who receives a W-2, he is self-employed and must file Schedules C and SE. On Schedule C he'll deduct his business expenses. If Schedule C shows $600 profit, his maximum Roth IRA contribution will be $600 minus one-half of his self-employment tax. That would be a maximum contribution of $557.
