gooday, try to tell him that other schools are also good and no admitting to DBS is not the end of the world.
Kids at this age can still be influenced by us. If we see this thing easily, they can still learn from us. It can be a good education opportunity.
Cheer up! The first thing do not let him lost the confident. He is as good as many others. School will not make someone super but a family does. Thinking of the 12 years ahead, with university to go ahead, this is just the beginning.