The way I see it is:
If your son is the type of person who will take advantage of their free time, he should do 10 units. The free time he is given will be used to study (for the most part) and he will get better grades in the 5 subjects he does than he would have gotten in the top 5 of his 6 current subjects. If, however, he isn't so self-disciplined, perhaps 12 units may be the better way to go. Having an extra subject will keep him busy, may make him more productive in his study, and can keep him out of trouble.