I don't think there's anything wrong with taking 2 BTECs and 7 GCSEs, as long as he is happy with the ongoing coursework- especially as some of his friends might have none at all!
Assuming he is thinking A-levels as a next step, most sixth form providers just want to see good grades in the subjects he has taken, rather than specific subjects. You could check with local sixth form colleges though as to whether it would cause a problem.
The thing about exam pressure is that some kids are fine with it, and some really struggle, and it can be hard to predict in Y8/9 who those children will be. Students taking lots of GCSEs can also end up with lots of days with multiple exams, which may impact their performance, but again, it depends on the student.
I teach triple science, and personally I think it is a good option for students who are bright but have one weaker science. If he does double, then his two grades are an average of all three sciences. Even if he is really good at e.g. physics, he is unlikely to get a 9 with one weaker science. IMO, something like 6,7,9 across three sciences looks stronger than something like 6-7/7-7 in double. I teach a few mathsy boys who do find biology a chore- but they cope and are still likely to get good grades!