1099-NEC is for services rendered to a business by a business person who is not acting as an employee of a corporation, so that's weird.

"Since the 1099s have not even been reported at all on his returns for the past several years"

They don't "get reported" like that. They are informational. The reason to issue a 1099-NEC to another person is because their services to your business totaled $600 or more, you want to include that as a business expense, and to "prove to the IRS" that it is business, you report the person you paid via 1099-NEC, when you paid them not through a third-party settlement agent (such as, you would not report if you paid through PayPal or VISA, because those agents will be reporting via 1099-K). If I do work for your business, and you pay me in cash, for instance, and it is $600 or more total in the year, you issue me and the IRS a 1099-NEC because I am not an employee of the business that just did that work for you. I am that business.

Reporting the payment as ordinary taxable income, is a bit of an assumption, as well. The gain from the sale is reportable, and the interest is reportable, which is the function of installment sales. But what if this was $500,000 sale, with $350,000 paid and $150,000 financed? Over some length of time, at some interest rate...or $2million sale, no down, and some sort of terms? Now, what if his basis was $500,000 in either example? See how gain is dependent on the facts, interest is dependent on the facts. You need to know the facts. The entire payment likely is not taxable.

Try this:

https://www.irs.gov/taxtopics/tc705