I agree with LoopyLoops. I'm an A level teacher in a borough where GCSE results are pretty low.
If a student gets an E at AS, it's very, very likely he/she'll fail overall as the A2 is much harder than the AS.
OP, you say, "Given the chance, statistically students do better in the second A level year and many do it over 3 years, with multiple exam re sits taking the best results forward to achieve the final grade."
I have to query this. Statistically, students DON'T do better in the A2 year. They might if they got a B and they are highly motivated to get an A grade. The problem is that for a student in that situation to get an A, they have to go up by two grades in the A2 year, so that the average goes up one grade, if you see what I mean.
Many, many students get WORSE results in the resit. A resit means that the student has to do the ordinary A2 work (which is harder than the AS) plus the revision for the AS, which they had struggled with, otherwise no need for a resit. Factor in the Christmas holidays, and the fact it's either 7 months or 12 months since the original exam was taken, and it's easy to see why so many get worse results.
They can't take several resits. In theory if they have a January AS exam, they can resit in the June, the following January and the following June. There isn't a teacher alive who'd recommend that or give that level of support. Support for resits means time taken away from students who are having their first (and usually only) attempt.
In my college, he'd probably take the two A2's and take another AS. He would then end up with 2.5 A levels, assuming he passed, which isn't enough to get him into university.
There are full time BTEC Nationals in Design that he could take - have you both looked at them?