Please, please, don't get your son onto Milo Yiannopoulis and Katie Hopkins or anything Claig recommends. That will just give him nightmares about shadowy elites, nebulous establishment bogeymen and evil liberal feminist conspiracies.
Try to introduce a sense of proportion and hope - I am worried about Trump too but it is not worth the anxiety you describe. Maybe try to get him involved in something positive? Show him how far we have come, and how the positive changes of the last few decades have come about- by passionate people not giving up. The scientific innovations, the improvements in equality and tolerance, working conditions, quality of life we enjoy now would have seemed unimaginable in the past.
A nuclear apocalypse is still very unlikely. A nuclear conflict may be more likely under Trump, but still very unlikely.
Trump is an nasty, impulsive idiot, but he is a nasty impulsive idiot who has just got everything he wanted - why would he destroy it all?
Even if Trump goes completely mad, he is surrounded by men with guns. Men who have families, lives and consciences, and will be aware what international consensus is on the "Nuremburg defence". Think about that.
In the (highly unlikely) absolute worst case scenario, it just takes one general disobeying orders or one bodyguard who values the world over the president's life to stop him.
Global warming is a threat, but it is a long term one. That is not to say it should be ignored - far from it, but a politicised, environmentally aware younger generation is a brilliant thing. Humanity has overcome greater hurdles, and if people like your son can use their passion in a positive way (point out to him that he will need his sleep and a good education for that) we will again.
