Take him to a psychologist. Not because he has a disorder, but because he is highly intelligent and both you and he need to learn how to deal with this gift.

Your son needs peers who share his intelligence. I don't know where you live, but any psychologist worth the name knows of local organizations that help highly intelligent children socialize with other highly intelligent children and find for them the special teaching that they need. Where I live, there is a special school for highly intelligent children. If such a school does not exist, your son could skip some classes.

Everyone – no matter if they are alcoholics or gay or highly intelligent – is relieved once he finds others who are like him. That is the single most important experience for anyone who is "different". The fact that you can talk to someone who knows how you feel is healing in and of itself. And making friends and having people that are interested in the same things will make him happy.

Do not send him to the internet unsupervised. He might be highly intelligent, but he still lacks the experience and maturity to deal with some of the more dangerous aspects of the net. There are places where nihilistic people gather and encourage each other in their negativity. Some of this turns to crime, some to suicide or drugs. Despite his intelligence, your son probably needs some kind of social life, and the lack of adequate peers makes him vulnerable, and his negative attitude makes him even more vulnerable.

Even if life is ultimately meaningless (which I think it is), you can still have fun with it. My argument to your son's argument is:

If you have two hours to live, why would you actively want to be miserable during those two hours? Purely from a logic point of view that seems like a stupid decision. You'll be dead long enough and there is no need to act dead before you die. Your body feels good if you provide it with tasty food. So quite obviously the fact that your body will eventually die, does not prevent it from experiencing pleasure. The same is true for the mind. All you need to do is find pleasant experiences for your mind. It is meaningless that life is meaningless. Life is what you make of it. You can choose to eat sand and feel awful. Or you can choose to eat (whatever tastes great to you) and feel well. I understand that with everyone being different from you and doing things that bore you, you might not know what kind of "mental food" you would enjoy. So maybe we can try to find kids who are like you and maybe you'll enjoy their company. It will cost you nothing to try, and you can go back to being miserable if you find that that's what you prefer. (Well, obviously you shouldn't say that verbatim ;-)

Give him autobiographies of other gifted people. Or do an internet research and find adults alive today who had been gifted children. Some have had difficult lives, others have found happiness. Show your son the adults and how they are not unhappy but have found something to do with their talent. Maybe some example of what your son might make of his gift will give him a perspective.

But most of all you all seem to be completely overwhelmed by the situation and need to get some professional support to help you out of it. It is common that highly intelligent children are depressed, but they quickly become happy once they find the appropriate peers and a schooling that addresses their talents.

A psychologist will test your son and find out what his special talents and what his weaknesses are. With this understanding he will help you tackle the problem.

Good luck!

Please read the section "Social and emotional issues" in this Wikipedia article: https://en.wikipedia.org/wiki/Intellectual_giftedness

Reply to your edit:

Visit the webpage of the MIT Department of Physics. Scroll down to the section "String Theory". Follow the links to the webpages of the people listed there. Contact one of them (maybe a native speaker would be best) by email and explain your situation. Don't write a novel, but do include some of the same detail as you did in your question here, explain your son's development, his behavior, your concern and include the quotes from your son. If you don't hear back from that person within a handful of days, they are on holiday or abroad (or not the right person for you), so contact another person. (Don't write to all of them at once.)

I work at a university, and many of my friends are academics in different fields, among them physicists. I can vouch that most of these people are highly social and very friendly and will certainly make time for an hour to meet with your son.

The people at MIT are people. They, too, have families and friends. I would be extremely surprised if no-one there would want to meet your son, especially since he might have real talent in their field.

But do not expect an immediate solution. That talk will help you to get feedback on the level of his abilities (make sure you have a few minutes with that person without your son, to get an honest assessment) and for your son to learn what I described above: that life can be fun through his talents.

But since the laws of your country probably don't allow your son to skip school and enroll at MIT right away – or, as Steven Gubkin pointed out in his comment above, your son might overestimate his own abilities –, you still need a strategy. If your son is not willing to see a psychologist (which I can understand), find one yourself or find whatever organization there is to help giften children in your area (or nationwide) and contact them and go there alone. Your son is not the first of his kind, and these people will have some practical ideas for the next steps you and your son can take. (If you chose to see a psychologist, don't visit some random psychologist. Call them and ask who they would recommend in your situation. Or contact your local university's psychology department.)

There are programs for giften children to visit university while still going to school, or there are special schools or after school activities, and these organizations will know the steps for you to take to get your son to where he will flourish. For you and your son to understand which of the many possible options are best, a psychological assessment will be necessary. You can explain to your son that this does not mean that anyone will want to push him from the path he envisions for himself, but to help him find the best approach to get on that path. Many professionals make use of tests to understand themselves better and better be able to manage themselves.

But he is still nine, emotionally, so you need to be the mother and nudge him a bit, if he is afraid or disinterested.

Note: Schizophrenia is related to high intelligence. For example, the twin of a highly intelligent person has schizophrenia more often than that of persons of average intelligence, and vice versa. The fate of your husband, who probably was highly intelligent as well, makes it important that you also consider the medical and psychiatric implications of high intelligence.

I'd be very happy if you would come back and let us know how this all goes.