These threads always make me feel like a crazy Tiger Mother, but whatever...
You are absolutely right to be shocked and disappointed. Because the following is not good enough, regardless of his academic ability, it is not good enough:
"He hasn't worked as hard as he should have"
"He admits btw he did no revision."
We are not talking about a student who worked hard and a D is the best of their ability. Of course that child should be hugged and supported and their parents should be very proud of him. But this student was a lazy bugger who did no fucking work! Why the hell would you bring comforting bowls of ice cream to someone whose laziness is squandering their talents?
He has fucked up massively, and he is big enough and bold enough to be told that.
Because if he comes back from this in the right way, it could be the most valuable lesson he will ever learn. It is all very well to have goals and ambitions. You can mind map all the success you want (I think that was a great idea btw). But it means absolutely nothing unless you are prepared to do the hard work to make those dreams reality. At the moment, he is all mouth and no trousers. Full of talk but not prepared to put in the hard graft.
And it is hard graft. Natural ability can coast you so far but that will run out and you will have to work. It is actually good that he is learning this now because (as you found to your cost in Cambridge, and at Oxford I saw the same thing) the older you get the harder it is to change bad habits.
He has the brains, if he wants to make the most of them he needs to be doing the work. A minimum of two hours a night every night from now until the exams. That is what it takes to achieve goals. Nothing is going to be handed to him. He needs to start working.
It is not too late. He can turn it around.
FWIW I do think you have fucked up too, by taking your eye off the ball with regard to school feedback and by sending him to an underachieving school in the first place, but I don't want to pile on in that regard because I think you know that and you don't need to feel worse.
Just remember, it is not too late. If he learns the right lesson from this it could actually be the best thing that could have happened to him.