As an ex-teacher I have come across this situation before and in the kindest way, OP, you need to have a serious look at your child and yourself before you go all guns blazing.
You say a member of staff witnessed your son slap someone, yet you don't believe it. Other children witnessed this, yet you don't believe it.
The primary school either did, or warned you they would, contact police about your behaviour towards them. What behaviour was this? A school cannot, and would not do this without good reason. Schools generally avoid involving police at all costs. It's only if it is really severe that they would ask for police involvement.
Your son was accused of bullying, yet you don't believe it. After years of your son being mean to others, do you not think they'd have had enough and started retorting back? Could that be why 'everyone' was against him?
A secondary school wouldn't start bullying your son just on the say of a primary school. If it's a state school, generally the only information that is passed on is attainment, attendance and any additional/medical needs.
Like I said, I have experienced this from a child/parent before so can see the other side. In this case, the child had learnt exactly what to say/crocodile tears to the parent to get them to believe every little lie. He was absolutely the instigator and the other children got to the point where they would say things back because they were fed up. Unfortunately in this case, the head was spineless and placated the parents instead of actually dealing with the child's behaviour.
Generally, you can get 1 bad teacher or 1 bad school. But if it's every teacher, every other child, every school, then you really need to look closer to home.