Tell him that my father, who was a maths teacher and Head of Department, used to worry far more about the children who came out all relaxed, saying the exam had been really easy, than he ever did about those who came out and fretted, like your ds. Chances are he has done better than he thinks he has, and the mistakes he thinks he made may well not be that major - but have blown up in his head into the End of the World.
I have said this to my dses, and have also told them to look at each set of exams as the gateway to their next step. As long as his AS levels allow him to get onto the A levels/A2s of his choice, that is what matters most, and he will be able to put all his efforts into getting the results he wants in the A levels/A2s - and then these will be the gateway to his next step - to the university of his choice or to the job or training he wants to do next.
All three of my dses have been lucky enough to get unconditional offers from the universities they wanted to go to - ds1 graduates next week - he got a 2:1 in Law, ds2 is half way through a 4 year applied maths degree, and ds3 starts his geography degree in the autumn. However, in part because they had unconditionals, they slacked off a bit during their final year at school, and neither ds1 nor ds2 got particularly brilliant Advanced Higher results (we are in Scotland - that is the exam that's the equivalent of A levels) - ds2 did particularly badly. Ds3's results aren't out yet, but we think he has slacked off too.
But we are not seeing this as the end of the world - sure, they are not good results, and will not look great on application forms, but as long as they pull up their socks and do well in their degrees (as ds1 has), that result is the first thing a potential employer will look at, not their school exams.
I am not sure if this is making any sense - I am full of a cold (such fun in this weather), but what I am trying to say is that each new, higher set of exams sort-of trump the ones below in terms of importance.
Sorry for rambling - I hope there is something in there that helps. 