@S C 

At this point, there IS no way to determine the amount.

POSSIBLY, the IRS will provide the info long before the 2020 return has to be filed.

If not, then you'll likely need to report that your *client*  DID get the full amount and hopefully, the IRS will correct if that's not the case.

If your client ends up getting nothing or less than he was "entitled" (?) to, it's his own fault for not keeping proper records.

At this point, there is not a darned thing any of us volunteer, tax professionals on this board can do to help.  And it's not really an issue right now is it?  No returns can actually be filed for a month.