I will start off by saying that I don't have a good answer for you.
There is no reason that a minor cannot earn income, and many do. I think the basic problem here is that YouTube is making up its own rules that conflict with the tax laws. I assume the reason that they won't allow a minor to have an account is that YouTube wants a contract to protect itself, but a minor cannot enter into a binding contract. There must be a way around this. Child actors and models, for example, have income in their own names. But how this works is outside my area of expertise. Some minors have businesses and the business income is their own.
To answer your question about Publication 929, there is no single source of guidance for child's income because basically a child's income is treated the same as an adult's income as far as taxes are concerned. The tax rules for a child's income are the same as the rules for anyone else. The main exception is a child's investment income, but that's not the type of income that you are concerned about.
Here's what the IRS posted on its web site in 2023 when they discontinued Publication 929. You will not find this satisfactory, because it does not address your situation. (This is no longer on the IRS web site.)
"For tax year 2022 and later, Publication 929 will no longer be used to explain the filing requirements and other tax information for individuals who can be claimed as a dependent on another person’s tax return and how to report and figure the tax on unearned income of certain children (whether or not they can be claimed as dependents). Instead use Form 8814 and Form 8615. Forms 8814 and 8615 and its instructions have been revised to include the additional instructional material in Publication 929."
Form 8814 is used to choose the option to report a child's investment income on the parent's tax return. Form 8615 is the "kiddie tax," which taxes part of a child's investment income at the parent's tax rate. Neither form has anything to do with any other type of income.
But information about a child's income is not your concern, because you are not really dealing with a child's income. Under the tax law, the income on the 1099-MISC from YouTube is your income, not your son's. You have to report it on your tax return. The people in the thread that you referenced suggested various ways of canceling out the income on your tax return and putting it on the child's tax return. I don't think you will find any of these methods in any IRS source. They are just various peoples ideas for working around the problem. I am not aware of any IRS guidance on how to handle the situation.
As an aside, it's not clear to me exactly what YouTube is paying you for, or why the payment is reported on a 1099-MISC. Which box of the 1099-MISC is the income in? Depending on the specific type of income, it might or might not be possible to deduct expenses. The $400 filing threshold applies to self-employment income. It's not clear that this is self-employment income.
