Yes, you can go back and claim him but you will need to mail in your tax return this year.
Your son will also need to amend his return by filing Form 1040X (also by mail). There is a good chance his return will have little or no difference as far as his numbers go - depending on his income, but he should amend the return anyway.
If he used TurboTax to file, he can just sign into his account and choose the option "I need to amend a return that has already been filed and accepted" and follow the prompts.
@haroldbecklin
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