Your post could have been written by me...in fact at one point I thought it was one of my old posts!! My DS has also head butted me so hard he made my nosebleed and I sued to regularly have to lock myself in my room to get away from him. Left a holiday early after he kicked and hit me so much I locked myself in car and he carried on kicking and thumping the car.
My DS now 6 was like this and thankfully he has calmed down. A few things that really helped were having a reward chart for home that was taken into school for the teacher to see. DS got stickers for being nice and not aggressive with so many meaning he could have a treat. It worked okay, not totally brilliant but it did help a bit. Doing exercise i.e. walk/park/scooter straight after school after a snack.
Having a day where DS got to choose what he wanted to do with bad behaviour ignored, lots and lots of praise.
Rephrasing how I said things, looking out for the triggers and trying to find another way of doing something. The AHA parenting site has some good tips amongst all the softy softy stuff that just doesn't work with such an aggressive child.
When a tantrum was coming on saying 'its time to calm down shall we get a teddy each and cuddle it on sofa/floor' sounds naff but help sometimes but you have to catch it right before its massively escalated. Then asking him if he thought his behaviour was okay towards me and how would he feel if someone behaved like that towards him.
He still has a temper but so do I and I am very shouty (loud) due to high stress levels which he clearly feeds off. I'm trying to relax a bit more and have been listening to hypnosis apps that help with that.
My DS is also intelligent and good at school. I did get help from them and it helped to have the teacher and the head teacher on board with any rewards stuff. They will have a behaviour person (can't think what they are called) and if you ask to speak to them they should be able to help you.
Its still a struggle and somedays I find it very, very hard and especially hard to find anything to give positive praise for but I do find the more praise I give the less the aggressive behaviour.
It gets better but I've been where you have been and its so fucking hard and I spent so much time in tears and wanting to give my son up. they have massive testosterone surges at this age which don't help either.
Big hugs x