The fact that he is your child won't help him visa-wise. As he is over 18 he is considered an adult. A carte de sejour is only a residency permit, but a visa is what you need to enter France for a specific purpose. To live in France for more than 90 days he needs to apply for a visa. To be eligible for a work visa he would have to be sponsored by an employer who is willing and capable, and the visa would have to be applied for from the US.
Alternatively he could apply for a student visa by taking studies at university, perhaps taking a French course, which would allow him to work a restricted amount of hours over the study period, but he would only be allowed to remain in France for the duration of the course.