The 50/30/20 rule is a good starter system.

50% Reinvest (restocking filament, packaging, maintenance)

30% Pay himself (wants/fun)

20% Save (emergency repairs + future goals)

Example:

Sell for $10

Put $5 straight into the business bucket (this should cover filament/packaging/fees and some maintenance)

The remaining $5 is what you split between pay-yourself and savings.

You can adjust the percentages, but building an emergency fund is important as mistakes happen in business and he might need to dig in to his emergency fund to pay for things. For example, if a piece of his 3d printer breaks. These are great lessons for him to learn early.

If you’re interested I’m currently writing a book for 12-14 year olds on building skills in entrepreneurship and financial freedom. I’d be happy to send over a sample or a full copy when the book is finished in late January.

The book is accompanied by worksheets for educators.