I am currently doing Edexcel maths and further maths and I can tell you that the only additional modules that it has are D1/2, M5 and S3/4. A lot of colleges don't run these modules anyway (M5 and S4), only specialist/public schools would usually run these. I have a friend who does OCR math (not further) at a different college, she still does the core modules. The content in the core modules does seem to be mostly the same with one difference. Her c3 seems to be my c4 and vice versa. So learning from 1 board and sitting another would be quite odd.
I wouldn't say there is ever a "best board" I think that any board that offers the modules you want/need is the one for you. The modules I am doing/done are C1-4, M1-4, FP1/2, S1 and D1. Although you say your son wants to do computer science at university and D1 would be helpful, I didn't find it very helpful. I did computing AS and from my understanding computer science is a lot like decision, but decision won't prepare you much more than other modules. Decision maths is also not to attractive to universities, it is often seen to be the easy option and doing mechanics or statistics may, not necessarily, help you in applications.
I don't know about other boards, but I would recommend Edexcel from my own experience. All the exams are incredibly easy and there is the vast range of modules. The text books are very good and the exams are very easy to 100%.
To sum up, if you find an exam board with the modules you want to do - sit that. It doesn't matter in the long run which you do.