You know, I've been thinking about this for a wee while toady. My oldest son really struggled in y11 due to pressure (no modular exams then) so maths and Science all one exam at the end, some coursework etc . He really felt under pressure, and it was all getting to him. The low point was his parent's evening that year (which fell on his birthday) and the last appointment was with the PE teacher (he loved PE). This teacher told us how disappointed he was in our son, and that if he really didn't turn himself around he wouldn't amount to much. I remember being absolutely gob-smacked, as we'd never really had any idea it wasn't going well. he's a lovely happy lad, usually, and I admit he'd gone a bit quiet, but I had just put that down to him being 15/16 and moody.
Anyway turns out he was really struggling with constant demands and work in things like French, and DT which were really bogging him down, and he ended up doing pretty well, to be honest (not in French
) . I'm kind of rambling here, but my point is that your son has a heavy lot of subjects there...perhaps he's getting bogged down in one subject, and that may be contributing to the situation, perhaps he's concentrating too hard on one because he's finding it more difficult?
One of mine only took 3 subjects through AS and A2, and it hasn't hampered her at all, better to do 3 well than 4 and struggle. Just a thought - perhaps when you have a word with him, you'll get to the bottom of it all.