Does my Thai citizen son need a visa if he entered Thailand on an American passport?

as was pointed out, take your son, his birth certificate, his mother, her thai i/d and the house book listing to the immigration office and he'll get a year extension on his american passport because he's thai (cost 1900baht)

There is no way to get him OFF the american passport an on to a thai one (once you get it) without him flying out and back in BUT that extension will buy you a year to fool with it so there won't be a gun to your head to do it soon 🙂

Believe me he's not the first thai who entered the country on a passport other than thai. Happens quite a lot

He entered on a US passport, technically he is an American regarding Immigration laws.

His choice would be:

1. Apply for a 1-year extension based on being a foreigner with Thai nationality: Go to your local immigration office and ask what he needs to show.

2. Get a Thai passport, leave Thailand by air on the US passport he entered with, come back by air and enter Thailand on his Thai passport. He needs to be registered his mother's house book, then get the Thai national ID.