I've had a Non-B based on marriage for many many years and I have never been asked for an onward ticket. Not once. But I have never been to the UK. I don't know why it would be different there.
Everyone in the tourism business thought it was a great idea to have 45 days. The Thai government and immigration police don't agree. The only way this will be implemented again (IMHO) is if the economy goes into a tail spin again. But you can still enter for 30 days visa-free and extend another 30 days when you're here so that's 60 days for 1,900 baht.
As others have said, there are plenty of flights, you could just book it for him when you are traveling together. Last minute flights are easy to get and the prices won't change that much. Right now it's cheap as chips to fly. Hopefully he can have a relative pick him up at the airport in Bangkok.
I see, good for you, that's wonderful. If you're worried about him you could just fly him back to Bangkok yourself then fly back to CR. The flights are only 8XX baht pp per leg. If he doesn't have access to a smart phone, travel agents still sell domestic tickets, you could direct him to one in his area wherever he lives. "Counter Tours" are never far away in Thailand. But as I said, you only need his full name (his name will be written in English in his passport if he has one or his driver's license if he has one) and Thai ID number and you can book it for him. Just use AirAsia, their App is quite easy or you can use their website and they have plenty of flight options. If he does the booking he can pay at
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, every Thai knows how to pay for things at "Seven" this these days.
Thai domestic flights are easy to book from anywhere, use an App by AirAsia or Nok or Smile or Bangkok Air, etc. If you put Chiang Rai to Bangkok into Google flights all the options come up. You need to ask him which airport he wants to fly into (BKK or DMK) and you need his full name and ID card number. Frankly it's easier just to have him book it and you pay him back when you see him. I don't know how you could make this a surprise if that is your intention. Flights are cheap assuming you mean May 23rd.