Unaccompanied Minor Overseas

My son will be going to France this summer to visit a friend. I've scoured the internet to figure out what is required for him to travel unaccompanied without much luck. From what I can tell, France has no requirements. However, entry into the US he will need to have a certified letter that he is allowed to travel either alone or with his host. Does anyone have experience with this? Is this correct? He will be returning with our friends but is flying to France alone. Most information on the US side that I've found is about children traveling with school groups. He is a responsible 15 year old. Are there any specific forms we need to fill out? We are considering getting him Global Entry to make the customs process easier. I'd appreciate any information from people with experience.

Thanks!