Bruises on his neck?! Jeez OP that is really serious. Is he being physically threatened on a regular basis? Have you identified which particular lesson is immediately preceding these 3 outbursts? (Is it PE by chance?)
You mentioned you're waiting for the school to assign him a counsellor (TBH I'd be happier to hear he was being referred to CAMHS - I had excellent experience with them for my DS's very troubled teens. He was also bullied and had abandonment issues with his birth parents. Please PM me if you'd like me to share our experiences with CAMHS.)
What have they said about the bullying? Have you asked for a copy of their anti-bullying policy? Are they following it?
A previous poster mentioned that it's a positive thing (IYKWIM) that he's turning his anger against his own possessions rather than other people or their things. You also didn't mention self-harm so that's another positive. He's taking his anger out on inanimate objects. Expensive, but replaceable.
Another positive that struck me is that he hasn't tried to lie about how the damage happened or blamed it on someone else. EG "I went to the bathroom and I came back and it was smashed, it must have been DS2" or "I tripped on my duvet and put my hands out to catch myself and I knocked it over". He has acknowledged and recognised that he has caused the damage through losing his temper - which means that he doesn't live in fear of you or his dad abusing him or withdrawing your love in response to his behaviour. That means he feels secure in your love, and that's a testament to you both as his parents.
Another PP (sorry I'm rubbish at namechecking) mentioned sitting with him and asking him to identify the "warning signs" of when he's getting on the path towards a blow out.
To this I would add, get him to think of things that would help him get off that path once he identifies it starting. Let him lead this, but you can make suggestions. EG "When I hear myself swearing at the screen/clenching my fists/grinding my teeth/walking with stiff legs/lowering my eyebrows, I will go and find Grandma/Mum/Dad and say "Please help me calm down, I'm feeling angry"
I don't know if DS2's status would handle this, but he could also perhaps help out? Could DS1 go to DS2 and say something like... "Bro I'm feeling really upset. Can me and you watch some YouTube videos/Tv/go and kick a football about/whatever." I don't know your DS2 obviously and whether this is something he could cope with, but I was just thinking it might help build bonds between them instead of antagonism.
Then you need to discuss what will help him to calm down. It could be inconsequential chat about nothing in particular, just to take his mind and emotions away from circling the drain of anger. Or sometimes it might be for him to actually talk about what's pissed him off at school today and how he feels about it.
Or it might be "Mum can we please go out for a walk round the block" or even simply "can I have a hug".
If you have a family pet, it could even be "I will go and cuddle with the dog/cat". The love of animals can be enormously healing.
I'm sorry this post is so bloody long! As I said, feel free to PM me any time if you want to talk in more detail or just share the frustrations of parenting a troubled teen. xx