There is no need to make it more complicated than it is. He is referring to a retirement visa—not a temporary residence permit—and the requirements for monthly income. Whether he chooses to apply for a Non-O or a Non-OA visa, the monthly income requirement is exactly the same.
There are only four types of retirement visas, and the requirements are generally consistent across the e-Visa portal, irrespective of the applicant’s nationality or country of application.
But he want to apply for the visa with income and only need to prove three months of income, unless he applies inside Thailand and his embassy don’t offer to provide affidavit letter for income.
When applying for a 90-day Non-O visa for retirement based on monthly income from outside Thailand—which is generally recommended—you are required to provide proof of income for the past three months exceeding the minimum required amount. Acceptable documentation includes the last three pay slips, a three months statement from your pension provider, or the last three months’ bank statements.
Embassy requirements vary. Most accept a short-term booking and an onward ticket within 90 days if requested. Some, however, require a booking for the full intended stay and an onward ticket within the initial 60-day period of entry. If a tourist visa is necessary and the embassy enforces these rules, the only option is to make a booking—which can later be canceled—and secure a low-cost ticket departing after 60 days.
If you choose to renew your passport either in Thailand or in the UK, you simply present both passports at immigration. You will be stamped into Thailand on the new passport and/or you can go to immigration to have your existing stamps and remaining stay transferred to the new passport without any issues.
Ok, The requirements listed in the visa portal may appear somewhat ambiguous, but in practice, they are normally applied such that you must document either three months of income or a certain amount of funds in your account at the time of application.
If you are applying based on a lump sum in your account, an updated statement showing the final amount, i.e. the "current balance", is normally sufficient.
A three-month bank statement or confirmation is usually required if you are applying based on monthly income. However, It should generally be possible to apply based on three months of payslips from employment income or three months of pension payment statements from your pension provider as well.
Ah, ok — then he should have applied for a multiple-entry tourist visa before coming, and he will now have to stay in a country outside Thailand for the required time to apply for a new single-entry tourist visa.
You don't have a Non-O visa. If you applied for a 60-day tourist visa through an embassy before arriving and then got a 30-day tourist extension at your local immigration office, you're good to go unless you're married to a Thai and can apply for a 60-day extension for visiting Thai family. Your eligibility for a border run and re-entry on a 60-day visa exemption depends on your passport nation. Consider using a professional land border transport company for the trip. Officially, there are no rules for how long you need to stay out, but some border crossings make up their own rules for foreigners who do a border bounce on their own.