If the total amount if income you will earn for the entire tax year will be less than $12K, then no tax return needs to be filed at all and no taxes are paid.
Otherwise, if your son runs this like a business and he keeps the money, (with you as a trustee of course), I have no doubt that your son's total business earnings will be less than $400 for the entire tax year, and he will not need to file a tax return at all. Here's how this works.
If your taxable income from all sources does not exceed $12K (24K if married filing joint) then you are not required to file a tax return.
If your taxable "business" income (i.e.; self-employment income) exceeds $400 for the tax year, then you are required to file a tax return. So if you have $500 of self-employment income you will not pay "regular" tax on that money. But you will pay the 15.3% self-employment tax on the business income that exceeds $400.
So in a nutshell, let this be your son's "business" where he earns the money, and so long as it's less than $400 and he has no other sources of income, nothing needs to be reported and you'll be fine.
