There is nothing more you can do but wait.
The "easy" payments are being made first, situations such as your son's I'm sure will be addressed sooner rather than later, but may take time.
Keep checking the "Get My Payment" app for changes.
There is no way to contact the IRS by phone.
If the payment is not received by the end of the year, he will claim it on his 2020 tax return.
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