I find it interesting that the school think there is a good chance that you will win. I note you mention a supplementary form which suggests this is a faith school.
An appeal panel is supposed to be independent. However, some faith schools use the same appeal panel year in, year out and they cease to be truly independent, taking their lead from the school as to who to admit and who to refuse.
As others have said, strictly speaking under ICS rules you don't have a case unless you can argue that your son should be excepted, e.g. because there are no places available at any school within a reasonable distance from home. The school obviously decided your son was not excepted when they refused admission but the appeal panel is entitled to review that decision and overrule the school if they think it was wrong.
You should also make a case as to why this is the right school for your son. If the panel decides this is not an ICS case they will want to know how your son will be disadvantaged if he doesn't go to this school.
In terms of preparation, you need to be ready to present your case. Don't just read out the written case you have submitted it. The panel will have read that before the hearing. Make the strongest case you can as to why your son needs to go to this school.
You will probably be asked to sum up your case at the end of the hearing, so have a very short (1-2 minutes) summary ready, although you need to be prepared to modify this if significant new information comes out during the hearing.
After the school has presented the case to refuse admission you will be given a chance to ask questions. Think about what you will ask. You want to use your questions to try and undermine their case where possible.
You need to be realistic about your expectations - this is a long shot. But you should make the best case you can and hope that you get a panel that is willing to bend the rules.