Hi, OP. Well done for getting this far.
I suggest that this afternoon you write out what it is you want a court to know.
When you have done that, go through it again and make bullet points of each of the separate issues. Take all this with you when you go to court so you remember what you want to say.
If you can, iy will be helpful to organise what you want to say into a statement. At the hearing, ask for time to organise your statement: I suggest 14 days.
Think what else do you have that an objective person would think supports your case. This means probably not letters from close relatives and friends but, as has been suggested, social care and any other organisation that has knowledge of what has happened.
In your statement, you should set out when and where your DS told you what your ex has said. Don’t add any comment but do explain the circumstances in which he told you this. Has your DS said anything to the school? Has the school said anything to you about your DS‘s behaviour?
Keep it succinct: judges don’t have a lot of time to read. Keep your paragraphs short. Be clear what you want the court to do. By this, I do not mean that you should have knowledge of legal remedies but you should be clear what outcomes you want.
I suggest that your last paragraph is a four sentence explanation of how this is affecting you and your son.
Judges are used to dealing with this sort of situation and with people in your circumstances.
It might be that the court has support from people who can assist you at court. For example, one local court I know, in fact two, had support from law students. They can’t advise you but they can help you organise your information and might be able to sit with you in court.