It would be WAY better for your father to get a year-long, multi-entry Non-Immigrant Type O-A (Long Stay) visa from one of the 4 thai consulates in the US that sell them BEFORE he wings his way here.
That visa is good for unlimited entries for the year validity, every entry gets you stamped in for a whole new year and he would show proof of funds by using his money in his bank in the US not here.
IF he used that visa correctly he'd get almost 2 years of stay out of a visa that is valid for one year.
It's the way to go.
Here's the thai consulate in Los Angeles' requirements for the O-A (Long Stay) visa
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