I think he can already live in Spain for the rest of his life if he wants.
At a conservative estimate there are 370,000 UK nationals residing in Spain. If you are one of them and you have been living in Spain lawfully for at least five years, you will be able to apply for indefinite permission to reside there, in Spanish ?permiso de residencia de larga duraci?n?. That will mean that you will be able to remain in Spain indefinitely provided you renew your residency card every five years (or 10 years in the event that you are a UK national or the spouse of a UK national to whom the 10 year ?permiso de residencia de larga duraci?n? applies).
https://www.solicitorsinspain.com/articles/living-spain-after-brexit
And it's not like he'll actually need to "work" again in his life, he can just retire under Spanish sun without needing a work permit.
In addition to the requirements for indefinite permission to reside, those applying for a right to live in Spain without permission to work need to show that they do not suffer from any illness that may be a serious public risk, such as smallpox, or Severe Acute Respiratory Syndrome (SARS), as well as others. Curiously, it will also be necessary to show a significantly higher disposable income than that required for indefinite permission to reside, in that the head of the family will need to show monthly income of at least ?2,259.60 and for each dependent a further ?564.90.
Yeah, you don't need much for permanent residency if you're non-EU in most of Western Europe really. It's usually 3-5 years of temporary residency across the board.