What prep are you doing with your DS - is just test papers? Perhaps you could vary the subject matter a bit - spot the difference puzzles are good practice for NVR and games like scrabble are excellent for VR. What I mean is, the learning does not have to be through a formal paper...
Have you tried just working on specific topics/areas within a subject - say converting units of measurement in maths. It may be that he is bored of doing tests where most of the questions are not bringing him anything new or challenging, iyswim.
Do you really need him to do 10-12 hours per week at this stage? That's an awful lot imo given that you've been going for a year already (if I've understood you correctly) and you still have some months to go.
Does DS feel pressured by the amount of work? It does sound as if, whatever the reason behind it, you should relax the regime for a bit. Good luck, hope he gets back on track and you can stop fretting.