You can keep the stimulus payment - your ex will still get the stimulus since he is claiming the child. Yes, you will need to amend and remove your son and pay back any credits you received for him. But as said before, you can keep the stimulus. The amending forms won't be ready until later this month so you are going to have to come back to do that.
Since you claimed your son, your ex will need to file by mail and claim the child. He will get his full refund + the missing stimulus that he did not receive (because you received it). Since amendments can be e-filed this year, your ex may want to wait to file until after you submit your amendment. Maybe he will be able to e-file once your amendment is processed. I am not sure if that will be possible, but it may be worth it to wait and see. Or, he can mail it when he is ready, but paper-filed returns will be delayed.
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