[ozzyowl1]

My son flew back from LA on his own at 17. He like your son had originally only wanted to go on holiday for 2 weeks, while we were going for four. His dad could only get 2 weeks off work so he was due to fly home with him. A couple of months before we left he changed his mind and wanted to stay the whole four weeks!
I called BA who wanted a ridiculous amount to change his flight, and ended up booking him on a separate flight the day before, I think with Thomas Cook. I had to phone to book as he was under 18 at the time but it was a direct flight so I wasn't worried about assistance, he managed fine but I don't think I would have trusted him on an indirect on his own, there is just too much that could go wrong. What if he missed his connection or the flight was cancelled?
If I were you I would try to get him to change his mind, (not easy I know!), or consider putting him on a direct flight from a different airport.
Good Luck.