Can my infant son enter Thailand with a U.S. passport without a visa?

He can enter visa exempt on his US passport, and extend by 30 days if required, no problem.

Since your son is using an U.S. Passport just like you, he will make Entry into Thailand the same way you are, unless you already have a Thai E–Visa PDF File or anything else?

My son was in that situation but from Australia. He required a visa but that was because we stayed 60 days pre 60 day visa exempt days.

If he holds a US passport he'll get in free stamp on the visa exempt entry program.

Right now that is for 60 days but it's been approved to go back to 30 days in the not too distant future.

So no problem entering Thailand on his US passport..

Make sure you file a TDAC (Thailand Digital Arrival Card) on both him and you as you two are entering the country as Americans. Your Thai wife is not required to file one

Here's the website

tdac.immigration.go.th/