Oh it is horrible to be a teenager, but it must also be horrible to be a teenager who is struggling's mum in this day and age too. The two do not cancel each other out! And you are here on mumsnet so you get to have sympathy for being a mum in this situation. And having to parent a highly reactive and responsive teen in a world designed for quick likes, immediate engagement and constant overstimulation is hard, especially when we know the pitfalls they are likely to encounter but we are roundly ignored.
You say school are supportive, but I hope that means they are supporting you with actual help not just sympathy. Like, I'm assuming they have already got him referred to some attendance programmes, mentoring sessions, the EWO if necessary? If he is finding school tricky, they should be putting plenty into place to support him and therefore allowing themselves to be the bad guys. Whoever is in pastoral shoud be giving support in whatever way he needs it.
I would agree with PPs, there is a lot of time between now and year 11 to settle down and get some GCSEs under his belt, but even if he doesn't there are other options (not that lots of mainstream secondaries will focus on that). But I get that it feels frustrating right now, especially if he has the capacity but won't engage with things designed to help him. I probably don't need to state this but I will- your child is not a simple reflection of you, you are separate entities. You can't control their actions, and there is an element of decision making as to whether he will even notionally engage in whatever help is being offered (i.e being driven to school). It is sometimes poor choice making, driven by influences of brain chemistry, but it isn't something that you can ultimately control- possibly you might influence it, but control it? No. If it were you, I presume you would have made a different choice in that situation, and so don't let yourself or anyone else make you feel bad. You made as many accomodations as possible to make it happen, and it was his brain that made him do what he did today. You couldn't really have done much more.
In terms of him being nasty, that's really difficult. Is there any way that you can resolve this bit and reconnect or have him in some small way recognise the impact of it? I think I would find that the hardest part, especially after I had offered a lift! Is there anyone else who could give him a nudge to help him start to repair? If not, is there something you could do to make yourself feel better? I always suggest wine or chocolate, but maybe a nice cup of tea with friends for the equivalent two hours he spent texting you? A long bath with extra bubbles once he has gone to bed with a extra saucy romance scene that would make him blush if he knew you were reading it? I feel like it is important to get a sense of clawing back something for yourself that he would not be able to spoil, or that links directly to fixing what he 'broke'- i.e. your enjoyment of the day for that two hours he absconded for and was sending you horrible things. If he can't make amends, then it's almost like you are making amends for him. Being on the recieving end of daily or regular bashing is absolutely draining. It's easier said than done, but try to do some self care, or allow someone to take care of you for a bit in whatever way feels good.
Finally, you said about stuff being undiagnosed, I'm sure you will be on it in terms of chasing diagnoses, but a friendly reminder to follow up with all avenues of support to get them and the support he needs. If he is in year 9 and truanting and absconding, that is definitely a huge safeguarding risk and school should be escalating this and not accepting it saying let's just try again tomorrow if you aren't happy with that. Be as blunt as you can stomach. Plus a GP could do a CAMHS referral, school could refer to a mentoring scheme etc etc. you could reach out to PAS too if you havent already. Forgive me if I'm teaching granny to suck eggs, but even if you just send off a couple of chasing emails it might make you feel less helpless.
It's probably all a lot more complicated than anything I've suggested, and there are a million reasons to not be able to do any of it, but please do try to do something nice for yourself even if he can't/won't. Parenting is hard enough without depriving yourself of a little win in the face of overwhelming need every now and then!