DTV is a 5 year visa - plenty of time to research the country. Also must be applied from outside the country and stay out of the country while under the consideration.
Certainly apply in USA and extend in Thailand. Age (min 50) and savings in local bank account equivalent of 800,000 baht (for whatever months required by local embassy) is really the only requirement; not the income. He may need an agent in Thailand to extend but at least he will have the visa already for 3 months.
Border officer does not check the previous stay 90 day report or TM30 …. Only immigration office inland when renewing or extending the visa which is irrelevant to DTV as you have to leave at next 90 day report in any case (visa valid for 180 days only).