Will he have to have a return(or onward) ticket, on arrival in Thailand?

 

It depends on where he's flying out of and which airline (I'll let the onward ticket experts comment).

 

I suggest he applies for a non-imm O Retirement visa in his home country. The application only requires you to enter a flight in and out within the 90 days but you don't need to show an actual ticket.

 

Arriving with a non-imm O means he can immediately apply for a bank account to deposit his 800k baht. If he arrives on visa exempt he likely can't open a bank account without paying an agent to do it (about 4500 baht)

 

Once his 800k baht is in his bank account for 2 months he can apply for the 12 month extension of stay for 1900 baht.

OP, good advice above especially if he does not have a Thai bank account in his name only. 

Passport country? He may be able to obtain the non O retirement using eVisa. 

Uk passport.  Thanks for quick reply

6 minutes ago, DanChang said:

Uk passport.  Thanks for quick reply

You did not mention if he has Thai bank account.

Regardless best option would be obtain the non O retirement while he is UK

That is done online eVisa OR

He can visa exempt and obtain non O at his immigration office and subsequent 12 month extension 

This attached thread outlines the process.

https://aseannow.com/topic/1282251-visa-exempt-to-non-o-retirement-dates/

He is in SPAIN, He possibly has a bank account, he was in Thailand  before about 15 years ago and he had a bank account then but he is not sure if it has lapsed. Regards Conrad(in Dan Chang, Suphanburi.