In ChiangMai, take your papers to the copy shop downstairs and tell then what you are applying for, they will know what pages to copy. And it’s cheap, 2 baht/page
Look on the website for the US consulate. There’s a form you can download for a letter they will notarize to satisfy the income requirements.
The facts of his situation might not lend themselves perfectly to the form of the letter, if he is going to rely on funds in the bank and does not have ongoing income from United States. But the result will be equitable. And no one is going to investigate the facts.
If you only want to stay three months, it doesn’t make sense to apply for any of the retirement visas. Just get a tourist visa. Retirement visas are only cost-effective if you plan to stay for years.