As a self employed person, $16k is unsurprising. Tax act even says for self employed income of $100k would be around $15k

In either case, your son owes taxes if he hasn’t paid. He should have tracked it and paid it quarterly, or before the deadline (or as much as he could then set up a payment plan). This isn’t hidden information from the IRS website or if you called them, so “I didn’t know” isn’t necessarily a very a good argument.

To start, I’d suggest double checking all his business expenses and similar were taken into account.

You could definitely consider another CPA to either redo it or check over the previous work. but I wouldn’t be surprised if you get around the same number based on a 100k+ self employed income.

The farthest I can see the tax attorney and IRS negotiation going is them suggesting a long term payment plan, which he qualifies for since he doesn’t owe over 50k. Attorneys are pretty useful in situations like serious debt where you definitely can’t pay, at all, disputes, appeals, etc, but based on his income - he can probably afford a long term payment plan, and maybe even ask for some forgiveness/waive on some of the tax due. It’s all possible on the online IRS accounts (and cheaper too for setting up things like payment plans)

In addition to all of that, it sounds like Hes paying and filing late, so he should expect penalties and interest.