Hi,
One of the reasons for your confusion is that A-level mark calculations changed last year (for the first time since 2000.)
Anyone receiving their A-level results this year will have done six units in total (3 in Year 12, 3 in year 13) and the total grade will be calculated out of 600 UMS marks.
Anyone receiving their AS-level results this year will have done two units so far, marked out of a total of 200 UMS marks, and will do two more units next year, giving a total mark out of 400 UMS marks.
The UMS boundaries are...
E - 40%
D - 50%
C - 60%
B - 70%
A - 80%
The boundaries for each unit depend on the proportional split for each unit, which varies by subject and by exam board. So, for example, if you have two units, one 60%, one 40%, the 60% unit will be out of 120, and have boundaries of 48, 60, 72, 84, 96 and the other unit will be out of 80% and have boundaries of 32, 40, 48, 56, 64.
Overall grade has nothing to do with the grades of the units, and is calculated as % of the total UMS mark, so for AS level the boundaries are 80, 100, 120, 140, 160 and for A level next year the boundaries will be 160, 200, 240, 280 and 320. (However for anyone finishing this year, the A-level boundaries are out of 600 and are 120, 150, 180, 210 and 240)
It gets even more confusing when the raw mark gets involved. There is not always an easy formula to convert one into another, as this is the point at which exam boards scale to take into account whether it was a 'hard' paper or an 'easy' paper. You can find the conversion for the particular year by finding the examiner's report for that paper on the web site for the exam board (however they are not out yet.)
In general, it is possible to turn marks around but you have to encourage your son to work smarter as well as harder. Beware taking so many resits that he hasn't time to revise for them - it just costs money and splits focus. In general, it's difficult and counterproductive to resit practical units so the best thing in Art might be to seek advice on how to maximise marks in the U6 units. I would also agree with the posters who said that in art it usually comes down to portfolio for admission to a foundation year anyway, so don't worry too much about that.
In general, the AS results act as a huge wake up call for many students and I have lost count of the number of students I've taught who have turned it around in the second year.
Don't worry, and I am sure you son's school will have a handle on it.