Thank you all so much! I came back to find a ton of considered, helpful responses. Lots to think about.
The set-up at DS's school (I believe) is that out of approx 300 students in the year, 40% are offered the chance to do triple.
He used to love science, but doesn't like this teacher this year at all and it's really soured his enthusiasm. He's very bright and able, but he's also not a grafter - at the moment - preferring to do as little as possible to get by. But he's also really competitive! And he's already started to say he'll feel like he's not clever if he's not chosen (even though I don't fully think the triple is right for him).
My only concern is that he has talked about being interested in psychology - along with being a world class footballer, so who knows how long this interest will last. But thinking ahead, I know this is already a very oversubscribed course at uni. I wouldn't want a double to lessen his chances.
And more importantly, I'd hate to think that if 60% of the year did double, we are saying they're all disengaged, naughty and not going to amount to much grade wise! A) I don't think that's true B) I assume the 60% will be streamed