In reply to JohnnyW:
I'd have thought that some of these options might be a bit long for a 14 year old on his first outing, even if he is fairly active and sporty. It takes a wee bit of time for the shoulders/back/feet etc to adjust to the demands of a long steady day on the hills, so I'd have been tempted to go for something shorter, or with more alternative escape routes if he's getting tired or the weather's bad. If your son is knackered halfway round the Cairn Toul/Braeriach plateau, there aren't really any bail out options, and even after you've reached the summit of Braeriach, it's a long walk up and over the Chalmain gap on tired legs and sore feet.
If you wanted to go for a Cairngorms option, why not go over Bynack More to the Fords of Avon Refuge as an easy start on day 1 , then you have a number of options depending on weather/ energy levels ranging from returning the way you've come, to going up and over Cairngorm, or heading up to Loch Etchachan and over Ben Macduibh, then onto Lurcher's Crag, Cairngorm or heading back down. The view over to the CT/Braeriach plateau from up there is pretty spectacular, which might offer motivation for future trips - there's even still a bit of snow sitting in Garbh Choire.
If he likes cake, you want to try and make sure you're back down in time to grab some food at the Mountain Cafe in Aviemore - there's nothing better than noshing through a big slab of their beautiful moist sponge knowing that you've absolutely earned it.
Whatever you choose, hope the trip goes well - it was my dad who sparked my love for being outdoors all those years ago, and my fondest memories of him now he's gone are our days out together on the hills.