Actually, all the other 'bad' behaviour mentioned is really no big deal. OP speaks of her DS and another kid 'kicking each other' which is more boisterousness than bullying, along with 'being silly'. This kid is 9.
The tearing up of the other kid's property is unpleasant but, again, it's expecting quite a lot of a 9-year-old to appreciate that this item was 'unreplaceable' (and, tbh, if it was some sort of paper thing that was 'ton in half' has no one heard of fucking Sellotape?)
Also, Op's DS was not acting alone, from what she said. At least one other child was egging him on. Is OP going to instruct the other child's parents to cancel his birthday party?
Finally, this incident happened in schoo. It's up to the school to impose what they consider appropriate sanctions. Not a bad idea for the DS to be told to send a written apology and some sort of gift to the other child, but the matter should end there.
Tearing up somoene else's book/picture/document is certainly unpleasant behaviour from a 9-year-old, but it's a stupid, knee-jerk mistake to project onto it any kind of calculated malice: a kid that young isn't capable of seeking out and destroying someone's 'most precious thing' to cause pain. It would have been impulsive unkindness, egged on by other children (and if the itme was really that precious, the parents shouldn't have allowed it to be taken into school. It could just as easily have been lost or accidentally damaged.)