Your grand father can apply either from home on thai embassy website or in Thaïland (i did in october 2025) but it's much easier and less stressfull from home.
Hé can get the 90 days Non-O visa with proof of the equivalent of
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0thb in his bank account. The only purpose of the 90 days is to give you time to open à thai bank account to mâture your deposit for 60 days before the one year extension. He xould use his revenues instead of
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0thb only if his embassy confirms it and they no longer do it.
Upon renewal if he has depisited 65 000bht every month(if he deposited
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0thb every 3 months he would be shy 2 months out of 3). Thoses deposits have to come from ashore (with proof). At the Immigration officer's discrétion, he could use lets say
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thb every month and à deposit of
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0thb. Again at the IO's discrétion. One more thing, he must be able to show 12 monthly deposit of 65 000bht not prior the effective renewal date but 60 or 90 days (not sure) before