Call the school secretary in the morning, and make an appointment to see the HT - should be possible within a day or two at the outside. Say what it is about, and say that you would like the HT to have spoken to the class teachers about what has happened so that he is up to speed with all the details before the meeting.
Take with you all evidence that you have - a diary of the incidents reported to you by your son, other evidence (I presume that you took photographs of the results of the assault you mentioned the other week?), dates of discussions with the staff etc. Report how your son is responding - is he withdrawn / anxious / tearful / unwilling to go to school / uncharacteristically aggressive? As you say this has been going on in previous years, any records you have of previous discussions would be useful too - or if you don't have that, then you could ask the HT to speak to x and y teachers as well because they have information about problems in earlier years.
Ask to see the school's anti-bullying policy (the HT should have one to hand) and explain why you believe that what your son is experiencing is bullying. Check that the school has followed the consequences for bullying outlined in the anti-bullying policy so far, and how they propose to proceed if the behaviour towards your son does not stop.
Be assertive, but not angry. The HT is the person in a position to change things in terms of how the teacher and deputy head are responding at the moment, but as the HT has not been involved so far there is no point in being angry with him / her.
As a PP says, if you are not happy with the HE response, then you can refer the whole matter to the governors, again including all the evidence you have.