are you applying for a 1 year multiple entry O-A visa? That is a completely different visa than a single entry Non O visa. That visa requires 800,000 baht or around £21,000 in a UK or Thai account. Or an income of 65,000 Thai baht or around £1695 a month provable income.
For what reason? You dont choose a visa as I keep saying. You choose a reason to visit, The system chooses the correct visa.
He needs also to upload the bought travel ticket. He cant show a return and choose a date on the system online calendar in 9 months time for a 90 day visa. The system will throw it out
According to the online application and the London Thai Embassy website, you need to have Legal residency in the UK or Ireland to be able to apply in the UK