I found Santander to be the worst of my accounts when I came to Thailand, they froze my card very quickly. Luckily I most of my money was in other accounts.
As Lynnette stated (and other people have said it to me previously) Nationwide seem to be the most lenient, so maybe try them.
I have extended my stay 3 times now and on average it has taken me 45 minutes.
You go to immigration and get a list of requirements.
You need to complete a very simple A4 form with questions like; name, passport number, date of arrival in Thailand etc.
You supply the documents on the immigration list and pay 1,900 Baht, wait a few minutes and you are done for another year.
I personally don't think it's worth using an agent for this simple task and immigration are helpful.
As for circumventing the law by using an agent to deposit the funds, this is a mistake IMO as it is illegal and people do get their Visas cancelled for this reason. It's up to the individual what they want to do.
I agree with Paul and Baz. We drove down to Samui a few months ago for a vacation and must admit a couple of weeks is long enough. I think Phuket has more to offer, if it is island life you want and you don't need to catch the ferry to explore other places. We live in Hua Hin and the beaches are not as nice, but it's a great location to go south to the islands or north to Bangkok if you get a car. Try a few places before you settle down.
I love Cyprus, but think Thailand is a wonderful place to retire. Good luck.
He can get a non O visa based on retirement inside Thailand if he meets the requirements. He needs to open a bank account in Thailand first and this is virtually impossible without a long term visa these days.
Ask the agency to open a bank account for him. The rest he can do himself at immigration.