Let him threaten. Legal action to try to recoup will cost him more than your payment! And you have his text confirming his receipt of payment for his own stimulus.
Are you talking about the current $1400 (and you received $2800 for you and your child)? Then it is surely yours. Take the money out of the card and deposit it into a NEW bank account in your name only. Read my most recent column for advice on that process.
And be sure to file your OWN 2020 return, IMMEDIATELY, declaring your child as a dependent — even if you didn’t have any other income but child support. You don’t have to wait until he decides to file jointly for 2019. In fact, double check the wording on your divorce decree. Since you were divorced at year-end 2019, I’m pretty sure you can’t be FORCED to file jointly by the judge’s ruling. And since you two are so estranged, it might be better if you file your own 2019 return. Ask your divorce lawyer about the wording in the decree — whether it says “can” or MUST file jointly for 2019.