In reply to Spagnal2012:
What a brilliant thing to want to do for his birthday, good on the lad!
Easy scrambles and wild camping, there's any number of options. If you/he are set on Ben Nevis and you only have a few days then you might as well limit the trip to that general area.
Check out UKH Route Cards for ideas:
https://www.ukhillwalking.com/logbook/r/find.php
A few to get you started:
Ben N and CMD Arete, clearly the way to do The Ben. Not really a great one for a camp half way and it's a big chunk of mountain to carry overnight gear on. You could I suppose camp near the Allt a' Mhuilinn but prob better doing it as a lightly laden day trip. https://www.ukhillwalking.com/logbook/r/?i=47
Ring of Steall, which is a brilliant big day with some quite exciting exposure but limited hands-on difficulty, and combines well with a camp in the glen https://www.ukhillwalking.com/logbook/r/?i=930
Beinn a' Bheithir via 'Schoolhouse Ridge' https://www.ukhillwalking.com/logbook/r/?i=819
Buachaille Etive Mor - the classic non-scrambling round https://www.ukhillwalking.com/logbook/r/?i=712
Beinn a' Chrulaiste via the Pink Rib is one of the best kid-friendly scrambles around - shame it's not longer https://www.ukhillwalking.com/logbook/r/?i=1262
And for something a bit different and more of an 'expedition', you could take the train to Corrour, wild camp near Loch Ossian or stay in the Youth Hostel there, and go up one or other of the hills around the loch. Beinn na Lap is the obvious straightforward choice. I'm not suggesting you do the full distance in this route, unless you want to make that the basis of your whole trip, but it points you to the right area at least: https://www.ukhillwalking.com/logbook/r/?i=1467
A couple of things:
At risk of teaching you to suck eggs, it's worth remembering that the terrain, distance, ascent, remoteness, and possibly weather all tend to be a big step up from standard Lake District days. He might be fitter than you in a racing-up-hills sort of way but you've already hit the nail on the head - kids do tend to have finite stamina, something to bear in mind on something like the CMD route. Probably also best, as I think you've alluded, to have a couple of shorter days in reserve (Pink Rib is a good one) in case energy flags through the trip.
Likewise with the scrambling: I won't say don't bring a rope, it's a personal choice. But on low grade 'hillwalkers' scrambles with kids, if you're thinking about using a rope then it's arguable that you shouldn't be there with them. The routes mentioned above are all technically straightforward but not always low-consequence (a slip from the wrong bit of the Ring of Steall could be serious for example) so only you on the day are going to know if it's wise to press on or not. I would save any scrambling with kids for a calm, dry day. https://www.ukclimbing.com/articles/skills/series/children/how_to_scramble_...
And a final thought, and sorry for getting preachy: it's probably best if folk aren't too specific about good camping spots, for obvious reasons. Just check out the map and you'll spot loads of possibilities.