The Department of Homeland Security has made it possible to look up your Electronic I-94 information online now, which includes being able to verify your I-94 was updated upon departure of the US.
You can visit the Get I-94 Information page here.
If you have any questions or issues (such as if you get Record not found), the FAQ, accessible via the link at the bottom of the page, is fairly useful, such that it is.
If you and your father are truly worried, take what documentation you may still have showing he departed the country on time with you. That is, unfortunately, all you can do at this time.
This part is still mostly true, however. If your record has not yet been made available in the system, or if there is some other unknown problem, there is no other manual process to verify.
