Communication is generally better than no communication.
I would be asking DS for his version of events (without prompting). Where is the truth in this? Maybe the school is not intervening because this isn't true bullying, or maybe the school is trying to be gently with DS due to his problems. Or, as in too many cases, maybe it's the other kid who is the bully, and who is trying to use officialdom to continue the harrassment.
There were times when difficult child 3 was accused of bullying, or attacking another child. When I sorted it out, it turned out to be a case where difficult child 3 perceived that the other child had been hassling him (sticking pins and other sharp objects into him when nobody was looking). The other kid denied it but was seen by other adults doing this. The school was told but refused to do anything because THEY hadn't been the ones to witness it. difficult child 3 was sent to the classroom in company with this kid (no other supervision) which is when difficult child 3 hit him. The other mother was advised by a teacher to lay charges against difficult child 3 - she told me she chose not to, because she knew that difficult child 3 has problems). But the truth never came out properly, at the time, due to failure of people to communicate.
What helped us - setting up Sixth Sense program. This is only appropriate if the child is autistic. But there are other ways to communicate.
I would be putting nothing in writing, but I WOULD be maybe asking the school to set up a meeting at the school with the parents. Doing it at the school makes it clear this is a school responsibility, it also makes it possible to have the meeting refereed and monitored for safety. That way there should be no later accusations of threats or inappropriate behaviour (some parents can be so inappropriate!). In short, it should keep everybody safe.
What is said in conversation is a lot harder to prove in court, and court action should not be on people's minds at this stage. By meeting the other family, you show that you want to resolve any problem (ie you are responsible); that you are open to communication; it also gives you a chance to work out for yourself what the other parents are like. If the dad is a big, blustering cop who throws his weight around, you can fairly quickly get an idea of which way the truth is skewed.
The family nearby whose son attacked difficult child 3 in the street (unprovoked attack) used the joint excuses that:
1) their son has Sensory Integration Disorder (SID) & and can misinterpret a touch as an attack, and will often lash out inappropriately but only after someone else touched him first; and
2) he's getting really good at defending himself because dad is a small man who had to fight bullies off and learned to hit first, so sonny is being taught to defend himself the same way.
At about this point in the conversation, sonny said, "And after difficult child 3 hit me and I belted him back, I rode around the corner on my bike and got hit by neighbour 2, but I left him with a bleeding nose."
At which point dad said, "Good for you, son!"
Meanwhile I was pondering why this boy on his bike had been the recipient of two unprovoked attacks, while HE was on his bike and able to ride away, yet the others were on foot and neither had witnessed the other's attack.
difficult child 3's behaviour pattern at that time had been to sit on the side of the road, wait for no traffic and then throw his paper aeroplane to see how far across the road it would fly. To run out and attack someone would have involved major task change - not likely. (I also later independently heard that neighbour had been attacked unprovoked.)
Sonny's dad then told us they they had changed schools for sonny, because he had been bullied at the local school. From the reports we've since heard, from other kids and other parents, this was the flip-side of the truth.
So whatever you do - beware the flip-side. If the cop insists your son is the aggressor then you may need to swallow it and accept that different point of view must prevail in each parent, but to go on from here - the school needs to implement your requests in the IEP AND keep these two boys apart.
Good luck.
Marg