Huh? What do you mean he gets monthly W2s? You get one W2 at the end of the year to report your wages.
And a W-9 is for 1099 income, not W2 income. The penalty that Turbo Tax calculates is just an estimated amount, It's common for the IRS to adjust the penalty and send you a bill or refund.
You can go though the underpayment form 2210 in the return and maybe see. Be careful not to change anything in the return. I would first save the return with another name to test in. Go up to FILE - SAVE AS
You might be able to eliminate it or at least reduce it. You can go to Federal Taxes tab or Personal tab, under Other Tax Situations and select Start by the Underpayment Penalties. You will answer a series of questions that may reduce or eliminate the penalty. It's form 2210
It's under
Federal or Personal (for Home & Business Desktop)
Other Tax Situations
Additional Tax Payments
Underpayment Penalties - Click the Start or update button
