The IRS has a timeline for these claims. If they assess a claim for unpaid taxes, they will generally send you several periodic notices. Then, after about a year of notices, they will file a case in tax court. As part of that case, they may file a tax lien if they so desire. It does not mean that he actually owes the money, just that the IRS says he does.
However, note that tax courts are under the Department of Justice (and thereby Merrick Garland) so he may not get a fair shake in this matter.
Yes, this didn’t happen overnight and he’s been working this likely for years to reach the point of a lien,
Two words....two words in the English language...”I Forgot!”