If he got Cs and Ds then he could have been doing either Higher or Foundation as there is an overlap. Yes, C is the highest he could have got on a Foundation paper, so if he is disappointed with a C, it is possible that he was sitting the Higher paper and expected to do better.
You need to check if he really is doing modular if this is his first set of GCSE exams because with modular (at least in maths and science that I know of) it's 3 modules over two years, not 2 sets of exams and he should probably have sat some modules and got some results before now.
He could also be sitting a mix of linear and modular, so linear maths, but modular science.
Some schools operate a weird system where they enter the kids for some of their GCSEs in Y10 and some in Y11, linear fashion, in which case these Y10 results would be his final grades for those subjects.
It's all quite confusing, I'm afraid, and changes from school to school.