I wouldn't go back to 2014, I would start with 2020 and work forward (if you are in California, you might eventually have to pick up years before 2020, but wait for them to ask).  I would not tell him that IRS will start wage garnishment, because they won't.  He has rights to a reasonable collection procedure.  I wouldn't put much faith in his stories about why he is a nonfiler, and I would collect the fees before I started. (He may say that he made a mistake paying in advance last time, but the mistake he made, if any, was with selecting the preparer he hired.)  

If he is getting a refund on 2022, IRS probably won't pay it until they get the previous two years of returns filed also.  

If he owes money from before 2014, the statute of limitations on collection may have expired. Don't do anything that would extend it.