Yes, I think he's old enough. We got our DD (17) one when she was about that age, and it's invaluable. She has her own plan, though, as we couldn't get a decent one that shared minutes. We pay for it, as long as she stays under her allotted minutes. If she goes over, she has to pay the difference.
I would monitor his usage (and his calling detail), though, if you can. We have Sprint and we can view it online. If there's any unusual phone numbers or times of day that are in question, you can then address it with him.
It proved very valuable for us when she went to a party last year. I had a funny feeling that sometime she was going to need it. And she did. There ended up being drugs and alcohol at this "party" and we had talked ahead of time about it and told her if that ever happened, for her to leave immediately and call us from her cell phone. She and one of her friends did leave, called us, and her friend's parents picked them up at a nearby 7-11. She said she was glad she had the phone, especially then.
Just set down some ground rules and you should be fine. PLUS, if you go to WDW and you split up for a few hours to go on different things, you can call each other to find out where and when to meet up (if you have free long distance and roaming charges, that is).