My son filed his individual taxes and didn’t know that we claimed him as a dependent. He did not choose Someone can claim: __you as a dependent. How can he change this?

Did he e-file it yet?  Was it rejected because you already filed and claimed him?  If so, it's easy.  he can go back to My Info and click his name and then go through the screens to the question that asks if he can be claimed as a dependent on someone else's return.  He needs to say yes.

If your return is rejected by the IRS, you can correct the problem and fix the reason for rejection and re-efile your return. If your return is accepted by the IRS, you must prepare, print, sign and mail an amended tax return form 1040X to correct the problem.

DO NOT change your original tax return until you start the amended return. If you have already changed it from the way it was originally filed and accepted, you must change it back before you start the amended return. Wait until you receive the refund from your original return before beginning your amended return to make sure you enter the correct amount. Here is an FAQ to help you:
https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed

An amended return cannot be efiled. You must print and mail it yourself. TurboTax does not print and mail it for you. In normal times, it takes the IRS about 16 weeks to process an amended return.