The school we appealed for always takes 10 above their published admissions number, and say in their submissions that beyond that number they can demonstrate clearly a prejudice to the other students by admitting extra. It seems that your LA are making a similar argument so effectively there are 9 places up for grabs.
At our hearing we didn't ask any questions about this but one of the panel members then asked the school whether, in the last few years, it had filled all of these extra places (which they had). You could possibly put this question to the admissions authority at the hearing, just to demonstrate to the panel that they can take the extra nine , although they are probably aware of this anyway.
Beyond this you will need to demonstrate why this is the right school for your son. In the advice notes we were sent they listed possible reasons as being:
family or friends and other links with the school
your views on education and behaviour
reasons to do with different subjects and the subjects your child is good at or enjoys
how close you live to the school or how difficult it would be to get to another school
other factors to do with the school
If you need to provide any further evidence regarding any of these make sure you take enough copies with you - probably 5 - one for each panel member, one for the clerk and one for the LA.
If the school has a specialism that is something your son has a real talent in, that might be worth arguing. Try looking through the school's prospectus or on their website for any other ideas about what makes this school particularly suitable for your son.
Good luck with this - you've got a lot of work to do!