Well, you've got a choice. You can spiral him into depression by telling him all that nonsense and that his life is over due to something that nobody even looks at[1] once you've got a job or A-levels (Maths and English aside, which you can easily get at night school and many people do), or you can give him a boost by helping him find a route forward that will allow his dreams and aspirations to remain intact, just delayed by 12 months or so (which is insignificant in what is likely to be an 80 to 100 year lifespan), or for him to find another route to them. Which is it to be?
If what you said is true, people like my Dad who utterly screwed up school would have been destined to live in a Council flat alone claiming benefits forever, rather than what actually happened which is that he took stock, took some actions and rebuilt it all and now enjoys a comfortable, early-retired life in a large 4 bedroom house in a nice area of Lancashire with a grown up family and (until COVID came along) several holidays a year. Not quite as posh as our Cheshire correspondent, and a Vauxhall Mokka rather than two Range Rovers, but definitely a good life.
A problem at GCSE level - any problem - is not going to destroy anyone's life unless they let it - it's so early on there's tons of time to fix it. As adults who have an influence over children, it's incumbent on us to help them find a way.
[1] Seriously - as long as you get whatever is the modern equivalent of a C or above in Maths and English, literally the only purpose of GCSEs is to get you to the next stage. It doesn't feel like it at the time, but if you're applying for jobs in your 30s with a degree and a few years' experience, literally nobody even cares about them.