My biggest advice is to document everything, and do it all via email, write dates times, what your son is saying take photos of the bruises, email school and ask for a meeting with the head and also ask for the senco to be present, I'd also take someone with you to so you are not alone, phone or email the local education authority and document what has been going on with them. In the meantime reassure your son that you are there for him, and that mum is going to do everything she can to get it sorted. I have a son with additional needs and whilst he hasn't been bullied by children he has by certain teachers and I have fought years with schools and now he is 15 and he knows that if anything goes wrong that isn't his fault (don't get me wrong he knows right from wrong and is more than capable of making wrong choices) but if he isn't in the wrong I advocate for him and it gets sorted, don't be afraid of the school/teachers. Stay strong xx