I have loooooong experience in academia and now the industrial side of my scientific discipline, and my take on it is this.
GCSEs are a key to the next stage. So are a levels, and really, so is a first degree. Some doors will close to you if you've got a 2:2 or a 2:1, for example.
But... What makes people successful, assuming they have a basal level of intelligence, is what I call "stickability." By that I mean the dedication and work ethic to keep working, keep practising etc and to take failure as a lesson, get back up, dust yourself off and keep going. It's not falling down that makes you fail, it's staying down.
I've got multiple degrees and qualifications coming out of my ears. All As, firsts and what have you. That's very nice, but it's really only gotten me onto the entry level of my chosen career. The rest was really hard graft! Long hours, and learning from my many mistakes, and I've had to move countries a few times.ma lot of sacrifice was involved.
I'm not privately educated, I went to one of the roughest comps in Yorkshire. My school was, quite frankly, a pit. No encouragement, poor teaching, basically crowd control rather than education. Parents didn't go to uni, or pressure me, but they did support me and encourage me.
Your sons next challenge is his A levels. If he can take the kick up the arse that are his gcse results, he can do great. GCSEs are really easy (sorry, they are) and I think a lot of bright kids fall into the trap of thinking that they don't need to work to get good grades. That's a dangerous attitude, and hopefully he's learned from this.