Help with a W4, teen first job

If you are claiming him, he doesn't have any exemptions, so I would have him do Single-0. If he claims 1, not enough tax will be withheld, because he would be claiming an exemption he will not have when he files his taxes.

Ignore the worksheet, just complete the form. Check Single, write 0 for the number of exemptions, and sign where indicates.

There are online calculators that do the same as the worksheet. Helping him do one of those gives him an experience that will be helpful later in life, so he knows what tool to use when his situation changes.

Just telling him to put zero and sign doesn't teach him anything about how to do it in the future, and putting zero so you get the maximum withholding/refund later isn't a good approach either if you actually should have a lower withholding. It reduces his paycheck, which could be money he could use throughout the year, especially when he gets older and may have a lowish income but expenses he needs to pay.

I say this as someone who outs zero and still has to have extra withheld each paycheck. It's better to learn what you're filling out than just what the answer is for now. There are videos on YouTube that explain what the form is and what the impact is when you out different numbers. Maybe watch one of those together.