He can apply for non-o visa at immigration and then during the last 30 days of the 90 day stay the visa allows he could apply for a one year extension of stay based upon retirement.
Requirements for the non-o visa application is here. https://www.immigration.go.th/wp-content/uploads/2022/02/9.FOR-RETIREMENT-PURPOSES-50-YEARS-OLD-NON-O.pdf
The problem he may have is proving his income for the non-o visa application. If can get proof of his income from his embassy it would not be a problem using the income option. If not then he would not be able to prove his income by way of transfers into a Thai bank from abroad to apply for the visa.
Even getting the extension application done can be a problem without proof of income from a embassy. He might need one year of transfers of at least 65k baht dependent upon how long he has been here and the office where he applies.