The months november to april pretty much cover the academic year - the last term (we're in the British system here) has only a few teaching weeks properly called.
My opinion, as a traveller and ex-teacher of Year 7 to 11 English and history:
Forget the distance learning. What sort of an adventure is he going to have working several hours a day? Travel is demanding enough already. Having to study along the way is barely possible, and even if he and you make the time and are able to achieve the massively high level of motivation and organisation needed, it will seriously detract from the trip. Total drag.
Very few students are able to study on their own at that age - it is a skill few people learn in the school system (why that is...well, don't get me started okay!).
Really, you are looking at skipping a year (no reason why not) or doing the Highers in one year rather than two - this is possible with some subjects, with a very dedicated learner.
Whatever negatives may be involved in these two options, they are hugely outweighed by the benefits of travel.
According to my maths, your son is just about to hit - or is hitting - what is for most people, the most difficult period of life. During those intense times, the promise of a family trip after Year 11 might just be the perfect way to motivate you all through the period, and pull you together after you've had your alloted share of rows and slammed doors.
Just a thought.
In other words sixteen seems a great time to go.
Last point, his teachers will be very lukewarm I suspect, in pubic anyway. It is their proffessional duty to discourage leaving the school system, so listen to their advice with this in mind. In private they may think it is great, but it cannot be said of a teacher that she suggested leaving school as a great educational idea, now, can it? How would that look when it got back to the HM? Not having to hold down a teaching job, I can of course.
So, no serious reasons why not that I can see, and some easy solutions. So go for it.