Firstly, I would get used to being accused of something by your ex's solicitor. They trade in accusations, nothing you do is going to make them stop making nasty allegations.
If you are not speaking negatively of your ex to the child and are not discussing strategy for getting your ex out of the picture. That will either be evident in the hearing or come out when Cafcass have time with the child.
I have found pointedly avoiding any negativity about my ex works wonders with my child. Children can find the negatives on their own steam.
As your son has been invited to the hearing, it is nonsense to suggest he cannot be spoken to about proceedings. Cafcass have a Young People's Board:
The Family Justice Young People’s Board (FJYPB) is a group of children and young people with direct experience of the family justice system. Its aim is to help make things better for children in the family courts. Find out more about the FJYPB and its helpful resources
www.cafcass.gov.uk
I suggest sharing this link with your son. It is a perfect source to give him some insight. The Board is made up of people as young as 8. This may sound weird, but I would consider inviting your ex to join you and your son for some child inclusive mediation and/or some sessions with a family therapist. Tell her that you would like to be on the same page about what is coming up so your son feels supported by both parents. Not that I believe for a second she will accept the invite. You would have tried though.
At 13 your son's wishes are likely to be close to determinative. Only pronounced welfare concerns should override his clearly stated wishes and feelings. This explains the solicitor's approach. Your ex may succeed in getting under his skin and turn him against you. Especially if he is living with her throughout proceedings. I would hope that a child of 13 would still want time with the other parent. Complete rejection of your ex, in the absence of some kind of abuse, might be a red flag.
This is dodgy ground and am not recommending it, but your son could choose not to be with your ex so much. An average 13 year old is very likely to be deemed 'gillick competent'. If there are allegations against you and your ex has a live with order. Court proceedings may be lengthy. That could be an ordeal for your son and, by association, you.
Just some thoughts. See what others have to say.