You are correct that your son would qualify for the Penalty Exemption for insurance being 'unaffordable'.
The penalty you are seeing is for his Dependent, who, since he is on Social Security Disability, should be insured with your State's Medicaid Program.
In the Health Insurance section, choose the option 'I'll pick who had it and which months'. Check all the months for the Disabled Dependent, and none for your son.
Then indicate that insurance was unaffordable for your son for all months.
You should get a message in TurboTax that the Penalty has been waived.
Here's more info from healthcare.gov:
https://www.healthcare.gov/people-with-disabilities/ssi-and-medicaid/
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