If your son's 1098-T for 2018 qualified you for an Education Credit (Box 1 amount larger than Box 5 amount), and you claimed him as your Dependent, you should Amend your 2018 return.
If he was not your Dependent in 2018, you wouldn't be able to claim his Education Expenses.
If you do Amend your 2018 Return to include the 1098-T, he does not have to amend his return also, since he didn't receive any credit and you were actually eligible to claim it on your return.
Click the link for detailed info on How to Amend a 2018 Return.
[Edited 2/7/2020 9:27 pm]
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