Welcome to the Forum. I agree with everything that Etienne and Guest55 have said.
The issue of wanting a specific school, or being more suitable for it, is not relevant at this Appeal.
Your primary task is to prove that your son is academically suitable for a GS, using the type of information that Guest55 has mentioned.
Your secondary task is to explain to the panel why he wasn't able to reach the qualifying mark. You should certainly speak to that expensive EdPsych to get a written opinion of how his dyslexia will have affected his performance in a VR test.
Please have a read of Etienne's wonderful "Appeals Q&A", which you can find by clicking here
. The Section that is most relevant to you is Section B, Appeals against Non-Qualification. I suggest that you glance through it all though, as there is other advice that you may find helpful.