Everyone bringing up child labor laws needs to realize this is his family's business. That makes things different:
The Fair Labor Standards Act's (FLSA) minimum age requirements do not apply to minors employed by their parents, or by a person acting as their guardian.
https://www.dol.gov/general/topic/youthlabor/employmentparentsMinors under age 16 working in a business solely owned or operated by their parents or by persons standing in place of their parents, can work any time of day and for any number of hours.
https://www.dol.gov/general/topic/youthlabor/exemptionsflsaThey are making me do bills with items that they bought for the store and I have to do them.
You're still not saying anything.
We're not asking what the bills are for, we're asking what you mean by "do bills"
Are you signing invoices for received products?
Are you clicking a few buttons to make an online payment for the products?
Are you writing/signing checks for the received products?
Are you putting in orders for products?
Are you responsible for managing the money with which the bills are paid?
Are your managing the entirety of business's books?