My ex says he filed an amendment, how do I know if he did? do I need to as well? he said yes & to send proof I'm to claim our son with mailing my taxes in.

[Opus 17]

You’ll need to tell more of the story.  Apparently something to do with a dispute over a dependent?

[MarilynG]

Yes, if your ex incorrectly claimed your son and Efiled his taxes, you will need to mail yours in to claim your son, as your efile will reject. 

For proof that he Amended his return, you could ask for a copy of his 1040X.

The IRS will pay both of your your Refunds this year, but will come later and ask for documentation on who has the right to claim your son.

If he did not, he will be asked to repay any incorrect Refund he received, whether or not he Amended his return this year.