"the program assumes that he received the $1,400 for his new child, which he did not."

Those funds are not named for each person. They are based on the census of that taxpayer's life. This was paid based on a projection of what the 2021 tax year filing would include. So, now you are reconciling. And it turns out that everything is fine. He got all the money he is entitled to, for his Actual Circumstances.