Good Morning... I am so very sorry for this situation... it sounds very serious at this moment, and hopefully the hospital will take the very best care of him...
You asked what the family can do to support him, and that is the best answer right now: to just support him. Fortunately, he is in a place where he will be cared for as he recovers... and as he does, the staff will provide the mental health care that he so seriously needs.
It isn't unusual for young people to reach a point of frustration and need outside intervention to deal with the stress of life, or some extenuating circumstance that they may be facing... he just needs the proper support by loved ones and training in new ways to manage the despair and depression that he is facing.
As you say he is still in a critical stage of recovery, the best answer I can give at this moment is to stay with him as the story unfolds of why he made the choice that he did. And take good care of yourself and the rest of the family while you face this terribly worrisome situation.
The worry that I have is that, in caring for him, my daughter will become exhausted, my granddaughter will be so distressed in her A level studies, and my grandson will STILL not want to be alive!
A very real concern... it will be very valuable to take shifts and be sure that everyone knows that he is in good hands right now, and to be sure to try to eat properly and try to sleep and just be gentle with yourselves as you slowly make sense of this. The answers will become clearer in time, as will the proper treatment. This is very difficult, no doubt... he is lucky to have such a supportive family to help him through.