Yes, that sounds right. You don't fill out anything in Part I at all. It seems like the IRS didn't think about your situation when they modified the 1040X form and instructions for 2018, although it is a common situation.
Depending on your son's income, 1040X line 2 might change, since there are special rules for the standard deduction for someone who can be claimed as a dependent. That would result in changes in some of the lines below that. It sounds like you are filling out Form 1040X by hand instead of using TurboTax. It would be safer to use TurboTax, to make sure that the standard deduction is handled correctly.
