Get a thai sim or esim and turn off data on your US sim. You should still be able to receive texts. The problem might be that you could also receive calls, which could cost you
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 🤷♂️