Well, it depends, since he got 39, its a really good score already. Saying that, even if he repeats it, he wouldnt have to do a lot of work throughout the year for it. If you have the confidence that you will exceed the 40 mark indefinitely than I say go for it.
But also, saying that, I did 6 subjects last year. To my mistake, lol. 5 at school so I had spares and 1 outside. 6 subjects is really difficult. You won't feel the stress immediately but when Sacs starts and dates sort of collocate together, you will feel the pinch.
Even when you get past that and all the homework. You will be pressed hard to spread your time equally and evenly between your 6 subjects when exams rolls around.
It is up to you, do you feel you can withstand the work load and try to make it into your top 4? Or would you rather concentrate on your other 5 subjects and leave your PE as a possible increment?
*As an after thought. I would also like to say I was extremely stupid to do a total of 7 subjects. When the calculation of my enter came around, my 40 for IT was excluded. LOL