If he enters on his uk passport he will be treated as a tourist. But being a thai national, I believe he is able to stay upto 1 year. Best thing to do would be to renew his thai passport once in Thailand, leave the kingdom and reenter on his thai passport
The difference is simply one is a visa that you pay for, the other is not any visa. But is a visa exempt entry stamp granting you the same 60 days the actual visa does. Over multiple entries, I think the visa exempt stamp gets scrutinised more than an actual visa entry.
and that being said, I left Thailand after 90 days and returned to the same address and didn't do a tm30 online based on what i read on various fb posts. That little episode cost 1600฿ in a fine 🤷♂️
I think the tm30 online is ok, until you need to show a tm30 to immigration. But as long as it is on their system, they just write a slip and fix that slip into your passport for free