I know he's stressed about school but honestly I can't figure out what to say or do that will help bring him out of this dark spiral. He's in such a dark place but I feel like everything I say to him only makes him angrier.
dreamer502 that is heartbreaking it makes me sad too but also it is good you are still able to be in close contact with him, and he wants something different. My daughter has had that feeling too, of feeling like if she killed herself no one would care. I can't usually say anything right either! She does trust my husband, he usually tells her to distract herself or to draw, she is very creative and has an outlet in her art.
This may sound like an odd quesiton, but does your son have a dog? Having an animal who depends on you and truly loves you can really make a difference, and dealing with a dog forces you to interact and learn from them. What if you went together and got a puppy or an older housebroken dog? A puppy will bond more but make messes.
Or, if not a committment, what about fostering a puppy or kitten from a local animal shelter? That is a shorter-term committment and also means you are doing something really positive for another living being.
If he doesn't like animals would he be willing to accompany you, with no judgement or big discussion, to volunteer with kids in an after-school program or a program for developmentally disabled youth? Helping other people is a very healthy boost to oneself, and it allows you to bond in different ways. Also the people you meet doing that maybe are a little kinder and less judgmental.
If school is stressing him out I generally would support a lower class load, or lighter classes, just so he has a successful experience. But I don't know your son or what his goals are, if that would feel like not being successful.
Like feldsparkle mentioned I hope you have a support system for yourself! That may make the interactions less tense too, if he doesn't feel like his success or failure determines your happiness.
Good luck! Let us know how it goes!