Op, I am going through this for the 3rd time (ds1 and ds2 at uni), ds3 sitting Highers (equivalent to some where between As and A2).
Firstly, I think you can boost his grades considerably if you can get him to work, and usually this actually means teaching them HOW to study. Reading the course book or notes isn't an effective way to learn, and with all of our DS, they have subconsciously gravitated towards studying the facts they already know (again, very ineffective as a way of increasing marks!).
This is what we did. And it works (grade rise from in most subjects from prelims to the real thing of 1or 2 grades).
Remove all technology including TV from the DC bedroom until the end of the school year. Seriously. Set up computer, ps3 or whatever in another room. The idea is not to deny them the use of these, but to set boundaries to help with self control. They SHOULD be gaming, watching tv, using the computer, or whatever helps them relax, every evening, but only after the agreed amount of work is done, home work is done, and not until 3am (sleep is incredibly important), except maybe on a Friday night! They really need the boundaries at that age if they don't yet have the maturity to make themselves do it.
Past papers and practice papers are the thing which helped all of our boys. But the important thing is using them to identify weak areas. Get the DS to do a paper, them mark it with them- discuss how their answer differers from the specimen answer, make harshly and look for topics that they are scoring very low in. Tell your DS it is a really GOOD thing to find those weak areas, that's where he will gain most marks. If your DC is getting 60% in a paper (for example) but you find a topic he's very weak on, some spot revision on that could boost marks nicely.
Also, exam strategies need to be practiced... ie use all exam time, to check you have read and answered the question, that the correct units have been used, + or - used correctly, that all questions have been answered (check exam paper for missed questions).
Honestly, I think many DC of this age have no idea how to study and it is so overwhelming that they shut down and give up.
I don't think it matters which path your DC chooses to take in life, university, college, apprenticeship or straight into the workplace, getting the best possible exam grades (for that person) is going to help them.
Good luck (I know how very difficult it can be)!