Form 8812 for taxpayer who was MFJ in TY2020, but now files single and received the ACTC

I'm sure someone probably already posted about this, but the search module is so terrible, I can't find it. And I'm not going to spend any more time looking. 

Facts: 

Client filed MFJ with his wife for TY 2020. They claimed 3 dependents and received the CTC. 

They received 5 payments of $750 each in 2021 for the ACTC (250 x 3 kids)

Client is now divorced and will file separately with only 1 dependent. 

I only know he got the advance payments in that amount because he looked at his bank statements from last year. He probably did get letter 6419 but says now he doesn't have it (suspect wife has it and they don't talk, so that's that). 

My issue now is I have no idea what amount to put for how much he received because the number they got is based on a different filing status and factors in 2 people and now he is by himself. 

I know IRS was sending separate letters to taxpayers that filed jointly last year that received the ACTC. They always seem to be for the same amount, so I assume they are taking total payments for the year and dividing them in half for each spouse? Based on this I would put $1875 since the total they got is $3750 (750 x 5) but I have no idea if this is correct. 

Or would I put the entire $3750? 

And yes, client tried checking on the IRS portal and was unable to retrieve any info after making an account.