You can actually capitalise on the fact that so much of the exam material is overlapping, I think - and okay, sightreading skills need to be developed for each intervals, but the reading-music bit of that remains pretty transferrable.
I'm with those who say - in the event of keeping everything going, rotate. I got LGSMD on really not that many hours' practice a week (I've posted this before, and by now, with the exam about two years and a bit ago, I'm likely to start exaggerating actual amounts - but certainly I was doing less than an hour a day on average, and definitely not practising every day for much of the build up time) - and did it by being efficient with practice. Also, a wee break now and again actually helps my brain to sort out gremlins away from the instrument. I do think that the idea of a session immediately after, or directly the day after a lesson is a good one, but every day? Not inevitably necessary at all.
I can't even begin to contemplate early rising for practice, but since your son can, yes, I'd try capitalising on it as suggested, and I'd also have thought that half an hour later to bed wouldn't be unduly stressful - but although having a predictable timetable might be good if there is a tendency towards ADHD, too much rigidity will both cause stress and leave absolutely no room for those spontaneous social events which our kids also need to develop well socially.
It's not the end of the world to miss an occasional practice session. A crammed life may make it necessary to do that, and there are benefits to having other activities interspersed with our routine.