I'm on the GCSE thread, and my DS is "supposed" to do well (although I'm having my doubts about that in some subjects). I wish you didn't feel that the threads are not for you, as it wasn't supposed to be about just the more able.
FWIW, this is what you could say to your DS to make him feel better: he has been working consistently and that it is much better than cramming or not doing any work at all (some say that they still aren't according to DS).
So that gives him a major advantage.
He got 4s which showed a big improvement. The hardest thing is getting started but now your DS is getting into the groove of working, he'll become increasingly more effective when revising, which may well have a positive impaçt on grades.
Remember that these exams are graded on a relative, not absolute basis. How he does against everyone else in year 11 up and down the country is how his grades will be determined, and with the way he has been working, plus the 4s already attained, it gives him an advantage.
It's sort of true, but if you can get him to buy into it, it will help his confidence, which will make a difference to how well he studies and therefore his results.
Then, just do the obvious: make sure he is physically ready: regular healthy meals, stay hydrated, S ome outdoor exercise, a decent amount of sleep.
Apart from that, you can test him from the revision guides, flash cards etc. Encourage him to do the parts he feels weakest at first in the subjeçts that he most needs and make sure he knows which topics are in which papers and on what dates.
That's what I would do and say to help your son, if he were mine. Mainly, I'd be looking to make him see that he has optimised his chances by the way he has worked and that has given him an advantage.