I think you kind of answered yourself why they would deny him. He's stayed in the country for 2 years and thought he could leave on a visa run and be allowed back in when he clearly isn't a tourist.
Assuming you don't mean Covid vaccines, you really don't need anything else. I don't understand why people think Thailand is some exotic place when most people are just heading to Bangkok or Phuket and would never think to ask the same question if they were going to Tokyo or Rome or any other developed metropolis.
There's no change of rules for Visa on Arrival. It was restored many months ago. The changes of rules are regarding Test & Go vs Sandbox and other entry programs which have nothing to do with also needing the appropriate visa to enter. The only new visa that is possibly changing is the STV (special tourist visa) that is good for 90 days. The normal tourist visa (60 days), visa on arrival (15 days), and visa exempt (30 days for most nationalities) does not change except incredibly rarely to add or subtract a nationality.