For each year that he gave more than $17,000 per person (this can also mean $34,000 to a married couple), he must file form 709. This is not part of a regular tax return, it is filed separately, and not included in Turbotax. It can't be e-filed, it must be printed and mailed.
Payment of gift tax is not required unless his lifetime total of gifts is more than $13 million, but gifts over the $17,000 annual limit must be reported to count against that lifetime limit.
Past year versions of forms are found here.
https://www.irs.gov/forms-pubs/prior-year
For gifts made in 2024, the form 709 would be due April 15, 2025.
There is a penalty for not filing the form, but if he catches up and files, and the IRS decides to assess a penalty, he can ask for a waiver for cause or general forgiveness.
