I believe one last step to what Brandon explained so precisely is that you need to get a re-entry stamp so your visa is not voided upon departing Thailand. I made that mistake.
my Thai wife has a bank account and we can transfer money to the account from the US using MoneyGram. Haven’t needed that for a few years. More importantly, my wife has the bank account app on her phone and uses the scan-to-pay feature. We do that all over Thailand and it is free, no fees. Brilliant. We never use or need ATM or bank debit card.
I was flying from South Korea to Thailand and anticipating getting the visa exemption on arrival. The airline demanded to see the onward flight. My flight from Thailand to the US was for 4 months in the future. The date was no issue as long as I showed the ticket.
You can’t add anyone to your visa. Your step-son would be required to get his own visa that is appropriate for his situation.
The US embassy states they won’t authenticate any document, but they in a roundabout way, will notarize a written statement that you write about your document.
I did that a couple years ago about my US marriage certificate.
Unlikely a Thai wife can go to the US unless she has a spouse or fiancée visa, a green card or US citizenship. Very long and difficult process for sure. My Thai wife got to the US because she outsmarted every person standing in her way and there were many. Left up to me, I would have failed to complete the task and I’m smarter than most people, just not smarter than my wife. Good luck.