Another vote here for the teacher and head teacher failing you and your ds completely (and all the other children in the class as it's bound to impact on them too in the course of the year if the other child is able to continue lashing out unfettered).
I would be on to the LEA and OFSTED in the morning to find out from them what you can do to force the school to do something. I would say that you will be putting a complaint into them because of the way that you and your ds have been treated, but that today, what you need is something concrete that you can say to the head and the teacher to get them to take your son's safety seriously.
I'd also throw the attendance thing into the mix when talking to the LEA, I've no idea if the head has signed your ds off with authorised or unauthorised absence (and it would be interesting to know exactly what reason she has provided!!) but I would say to the LEA that you do not want to keep your ds off school, you are worried about his attendance, you want him to be in school, but that you are not prepared to jeopardize his safety any further given that it has been continually compromised and the attacks seem to be escalating.
Seems that most head teachers worry about attendance. Depressingly, it might be that one of the ways that you can force the head to do something is to threaten show how your ds's absence will have bad effects on her absence figures.
I would also be pushing for the other child being moved to a different class (unless the other class teachers are nicer or more effective than the one your ds has). Or threaten to do something dramatic like say you are going to go to the police and ask them to come and talk to this child to make him realise that he must stop hurting other people. Or see if you can get some sort of order on him whereby he mustn't come within 20 metres of your ds (I know, those sort of things are probably only for adults, certainly not for young kids, but it is the sort of thing that might make her sit up and realise how serious you are - and if you get that sort of thing going, has the potential to make her life very difficult and so would be much easier to sort it all out now).
Have you managed to talk to any other parents in the same class as your ds - do you know if anyone else is having a problem or is this child just picking on yours? And do you know if he was similarly behaved at pre-school or nursery or is this all new to him? Just wondering if he has a longer history of being violent to other kids you might have more success in talking to the teachers about him...
I hope you managed to have a more successful meeting with the head this afternoon and that you have managed to come away with something!