He's worried double cert science won't be as challenging as most of the top set kids will be doing single sciences.
The syllabus doesn't change depending on who is doing the course, so doing combined doesn't miraculously become easier.
In the 'old days' of triple being done in the same time as combined I'd agree that there does become more likelihood of more able scientists doing triple so maybe the top grade 9 issues not being discussed so much. But I suspect that is less of the case now that it more generally needs a slot.
The triple course isn't de facto harder, it just has more topics. (DD has switched from doing combined to just biology, the extra stuff is no harder than the content already covered.)
However if expecting to do science A levels I'd think hard before not doing triple, as it is likely triple knowledge will be assumed as a starting point.
DD was also doing both Drama and Food Prep&Nutrition.
Drama - the thing to watch for is group work. If you DS might have any issues working with others then avoid it. Otherwise it makes a good movement break between other more sedentary lessons.
Food - has a good mix of practical & theory and is a useful life skill too. Some of the theory is really quite sciencey. Will require practicing outside of school.
Drama & Food both only had 1 written exam which would have made summer of y11 a bit lighter. The exam schedule can be brutal for those doing a full set of traditionally academic subjects.