Bigbluebus Given your son is now 16, why don't you let him do some research about where he might like to go? You shouldn't dismiss ex-polys out of hand, often they will have areas of specialism which they are very good at. Subject specific (rather than general) league tables are a good place to start- as well as researching courses online. He should chose a course which matches his areas of interest closely.
As has been said, for many science careers, you will be looking to go on to do post-graduate study after the degree. The most important thing is not where you did your undergraduate, but getting a 2.1, which will ensure you can get onto a wide range of post-graduate courses. The best way to ensure this is chosing a university where he will be happy, and will enjoy the course, rather than just chosing somewhere solely by ranking.
The Russel group is a research lobby group. Being a member of the Russel group does not guarentee a good student experience, high league table rankings or good graduate prospects in your chosen career.
As an example, Lancaster is a very good university (top 10/20) for many STEM subjects, that is not in the Russel group. Surrey is also very good for engineering, and again not in the Russel group. Bath is also top 10 in these areas. However, as they are near the top of these fields, they will not be easy to get into- it would be easier to get into a Russel group university which doesn't specialise in these areas. However, you would not get such a good student experience.