Don't know how it goes in the USA, but down here a strait out company ( PTY LTD ) is the easiest way to go
Partnerships become very complicated and have a massive amount regulations.
Sole traders are the easiest way to start a business but does not have all of the protections of a LLC.
There might be an age limit as well, down here sole traders need to be 14+ , Directors need to be 18+
OTOH you son probably does not have a lot o assets to loose, but you might be deemed responsible.
You really should have a chat to your accountant or solicitor before you do anything that can backfire latter on.
I have heard of a pair of sons sueing their father for under paying them & not paying superannuation to them .
He bought a shelf company because they were too young and the shelf company employed them.
We have minimum 2hr shifts so they needed to be paid at least 2 hours every afternoon when they mowed after school.
So get some qualified advice apart from what you get here.