We are Americans and travel to Bkk about once a year and a couple of years ago my son was flagged for frequent visits even though it isn't the case. He was told that he needs to get a visa next time. So the last 2 times he got a visa from the Embassy. He hasn't been in BKK for about 1.5 years now. My question is, does he need to get a visa for our trip next week or will he be able to get his visa exempt stamp at the airport?
Thai immigration rules are not clearly defined or properly published and often in flux. There's plenty of speculation on the internet, but you won't be able to get a clear and reliable answer on this, even if you personally ask an immigration official. Similarly, embassy/consulate staff would be next-to-useless, since they're under a different ministry than immigration at the borders, and can't even access the immigration database. What is clear is that over the last several years, Thai Immigration has taken a bit of a dislike of frequent visitors.
I would keep getting the tourist visa prior to any future trip -- it's a long way to risk a rejection at the airport. Note that in addition to applying in person, you can also get a tourist visa for Thailand by mail at any consulate.