What are my uncle's options for visa extension in Thailand if he can't provide an income affidavit and lives separately from his Thai wife?

Cash requirements do not change but now when he extends visa he will need to provide Proof of Income

When is his next extension due because the Embassy income affidavit is valid for 6 months so one obtained before they cease can be used for an extension up to November?

I thought you had to live together hence the pics you have to submit with application. 80 something have thought about that so he ain't the only one getting older. Listen to the guys who know their stuff on this site.

An agent will do yearly extension for around 15k baht. It's the cheapest extension that agents do atm. Don't wait last minute. You have to apply with agent at least 1 month in advance.

He needs a Thai lawyer

If he brought in 65k for 12 months he’s fine otherwise it’s the 800k but if wasn’t in all year they probably won’t accept it a marriage extension only requires 400k in the bank for two months then once approved he can spend it after 20 years he must of learned something but if there living in different houses he’ll have trouble as immigration does home visits to make sure you live together and they want witnesses and neighbors to answer some questions

Speak to his wife,

Needs to change retirement to marriage.

It may however be very difficult to provide evidence of a genuine relationship.

How long ago was their marriage registered.

he should move in with his wife, and get a TM30 registered on Immigration in this location. Then he should put 400,000 THB minimum in his Thai bank account and when the 1-year extension is due, apply for a "change of reason" from "retirement" to "married to a Thai wife", using as financial proof the minimum of 400,000 THB in his account, it needs to be seasoned at least for 2 months on the day of application. He will also need a freshly printed on the Amphur Kor Ror 22 marriage registry in order to prove on Immigration that their marriage is still up and active. Immigration will issued the 1-year extension "under consideration" for 4 weeks, and during this period they will do a home visit (that's why he must live with her, at least for a month or two before and after the extension application). Immigration will ask neighbours if they are living together, so this can backfire if he they live in separate places. After the 4 weeks, after the extension has been granted, he can theoretically take the deposit out of the account. In the next year after this, as soon as he has collecteded 12 months of transfers of a minimum of 40,000 THB, month for month not missing a single month, and prove that each transfer came from abroad, he can use the 12-months bank statement as proof of finances for the application to the 1-year extension of stay permit based on being married to a Thai wife

If your uncle is not living with his wife then he can't apply for an extension based on marriage. He'll have to stick with retirement. His options are either 800k baht in a bank account in his name for 2 full months before applying for the extension of stay, which must stay in the bank for 3 months after getting the extension and never dropping below 400k at any point OR he shows a full 12 months of overseas transfers into his Thai bank account immediately prior to applying for the extension. If he can no longer get the income certificate these are his only options