OP work for a langauge company and I help manage their summer camps in Spain that has kids coming from all over the world into Madrid from the age of 13 ... they all manage it perfectly fine and we deal with literally hundreds of teenagers flying in each week in July.
The issues we most commonly come up against are immigration issues (needing to show visas/not knowing the answers to basic questions like "Where are you staying? Who are you staying with" if they're asked during the passport control checks which flags up as a concern), delayed flights, missed connections and lost luggage. Fortunately with a direct flight you won't have to worry about connections but the other three issues can be minimised if your DS is appropriately prepared.
When you drop your DS off at the airport, if he's checking bags in then you should be able to ask the desk agent what gate he is departing from so that you know he knows the right information.
Then take him to security and advise him to go straight to the gate area once he's through (and make sure any liquids/electronics are packed to be conveniently removed from his hand luggage for the security check) but to keep an eye on the departure screens in case of any gate changes.
In case of delays I would advise him to firstly message you to let you know of the delay but also go directly to the boarding desk and speak directly with the airline staff and they will more than likely keep an eye on him to make sure he is safe even if he isn't on their official UM program. Also I'd make sure he has enough money on him so that if he is delayed for a while he has enough money to get himself a meal/drinks/snacks as airport prices are extortionate at the best of times.
Also I would encourage him to let the flight crew know he's flying alone, that way they can give him any extra help - eg he might not be able to reach to get his bag in/out the overhead bin, or he might be sat next to someone that is making him uncomfortable (whether it's a rowdy group of drunk people or someone who is behaving inappropriately) and they may be able to re-seat him closer to their station if they know, or if he did become ill on the flight they'd know he was alone and needed to be more carefully monitored/checked on than a child who is travelling with a parent/carer.
Make sure he has some record of who is meeting him - their name/address/contact information - and any relevant information about the trip in case he's asked during passport control, also what to do if his luggage gets lost.
In addition it might be a good idea (if he doesn't already know the language) to make sure he knows a few basic phrases of the local language - so that he knows what signs to look for in the airport and also so that in case of problems on arrival he can find an official and be able to say "Hello, can you help me?/do you speak English? I need help.")