Get yourself and your DS a big bottle of Bach Rescue remedy...great for stress.
Do you or do you know someone who knows how to use mindmaps? This is one of the most effective and least stressful ways of studying and there is an element of fun (and chaos) in it that boys may be drawn to.
I teach Geography GCSE and in my experience boys don't tend to like or do very well at the coursework (which is a shame as it is guaranteed marks). They might not even do that well in mocks. But they are usually the ones who get their acts together and surprise everyone with their final marks.
Does he want to go to college and need a certain number of A's etc? Because quite frankly I think the pressure to get A*'s is ridiculous. If he gets the minimum grades he needs, that is all he needs isn't it? But it is good to aim as high as you can I suppose to ensure you get what you need.
I suggest you contact all his teachers (or did you get this at parent's eve?) and ask what he needs to do for all his coursework to get up to date. Then help him to work out a schedule to systematically catch up.
In our school Y11 pupils are expected to work 70 mins per week on each subject. This might help. Working in chunks of 20 mins with 5 (ONLY 5 ) mins breaks has been shown to be most effective. If he is behind he may have to do a bit more. And after an hour a 15 min break.
In my own experience (I have zilch self discipline) it is the getting started that is the hardest. Does he have an environment that is condusive to study?
And see if you can get him to talk to you about it..we have a boy in our school who ended up quite depressed because he got so behind. We sort of gave him amnesty and a period in which to catch up. He is now doing much better and seems happier in himself too .. which is what really concerned me more than the coursework TBH.
Good luck.