My son shows as insured on my 1095A however it wasn’t till I filed my taxes that I discover I cannot claim him as a dependent. What should I do next?

You will have to amend the tax return that was filed to remove him as a dependent.  In the section for entering a Form 1095-A you will need to indicate that someone that is not claimed on your return is included on the form.

If the return is accepted you will have to amend your original tax return. An amended return, Form 1040-X, can only be printed and mailed to the IRS if the original tax return was not e-filed. The IRS will take up to 20 weeks or longer to process an amended tax return.

Before starting to amend the tax return, wait for the tax refund to be received or the taxes due to be paid and processed by the IRS.

See this TurboTax support FAQ for amending a tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-change-correct-return-already-filed/L4VjJ9BA2_US_en_US?uid=ld1n99kr