He can get an ITIN instead of a SSN.
If he does not want to apply for an ITIN, you can prepare your "Married, Filing Separately" return as follows:
You can't prepare and print
a tax return in TurboTax without an ITIN or SSN for you, your spouse and any
dependents.
The work-around for this situation is to use "Dummy" SSN/ITINs for your
dependents to prepare the return. You can use your own SSN/ITIN
with different last numbers.
Or use "305-44-0001, 0002, 0003 etc.", a dummy number
we use for training.
This should allow you to get through the Smart Checks and
Reviews and print your return for filing by mail. "Erase" or white-out the dummy SSN/ITINs and replace with
"NRA".