Exclusion from secondary

I really need help regarding exclusion, my son is 13 and was diagnosed with ASD as well as other things in 2011. on Wednesday my son was in the canteen having lunch when two boys came up to him and took his fully equips pencil case and started throwing it to other students saying free pencil case and pens, my son tried to get his case back but it got thrown to others in the canteen anyway a teacher walked in just as he put his arm around a student who had his case to grab it back but the teacher thought a fight was starting so shoutedat my son and gave him a red card, he then tried to tell her his pencil case was taken and he wanted it back to which she ignored and told him to leave the canteen which he did do but went back 5 mins later as he presumed she had got it for him, she had not got it nor questioned anyone about it so he asked her if she got it but the teacher turned and walked away from him which by now he is very very upset and shouting loudly he wants his pencil case back (he has followed her). Next thing is the head is speaking to my son as my son is being verbal ( frustrated no one is listening and getting his case back), my son was removed from the area and after 20 mins they found the SENO to speak to him. My son phoned me from his phone and was crying asking me to help him, I went straight up the school and once he seen me he was hysterically crying saying no one helped him, the head told me to take him home to calm dow and bring him in to school the next morning and he would sort a meeting out. The next morning I pulled up to school as I did the school phoned my Mobil and said do not bring him in he has been excluded for two days for verbal and threatening behaviour. I have a meeting on Monday with the head SENCO and my son about this incident and they want to know why he got like this? 

I could cry he has never ever been like this but he has never had anything stolen from him, he did have a teacher he could go to when he felt upset but she left 8 weeks ago so he has no one to go to now.

my concern was that this situation escalated quickly when it should have been dealt with immediately and not left to get to this stage, The school also said on the exclusion letter that it was not tolerating his anger but this situation was and the way he reacted was due to his disability and a behaviour linked to this.

any advice would be fantastic, oh and nothing has been put in place for him since the teacher left.