If you do get a choice between places, then to choose between them you can ask the schools questions like:
Is there a STEM club? (answer will probably depend on which year group - its more likely in the younger years).
What proportion of students do Triple Science as opposed to Combined Science at GCSE? Can students choose Triple Science if they want? (combined science covers all three sciences but is worth 2 GCSE's not 3 as the curriculum is reduced. Some schools only let a small number do Triple, and some even make them do the extra as an after school activity!).
Is Computer Science a GCSE option and, if so, can it be chosen alongside Triple Science? (Some schools, like Tiffin, try to limit the amount of science options taken, to encourage students to do art subjects).
Are there any other STEM subjects offered at GCSE?
Does the school participate in any STEM outreach programmes or competitions?
You can probably get the answers to some of these by looking at school websites, or emailing them.
For what it's worth, there are kids in interested in science at every local comprehensive school so I'm sure he will find his crowd.