This seems like more of an Air France issue not a Thailand customs issue. This is a complete overstep of airlines authority. I find it absurd and you may have found resolve going up the ladder. A gate agent to you that you could travel with your child!
I have never been stopped at a border traveling with a child, 30 countries and counting. In the U.S you need both parents consent or a court order to even get a passport for a child.
Without just cause, customs agents aren’t digging into parental custody issues or other countries bureaucracy around it.
Things could be great for him. If he has established credit in the U.S. He could maintain it from here or liquidate it before he’s deported. 5-10 grand can easily cover his living expenses for a year. With his language skill, he can setup a business catering to English speaking foreigners Or get a decent paying job.
His position in my opinion could suck but presents great opportunities.