You must also remember that you are not appealing against the school you have, but for the school you want.
If Year 2 and Infant Class Size rules apply, then they cannot breach the limit of 30 per teacher, and the only grounds for winning and appeal are
a) mistake in the administration of the application, and if the mistake had not been made your DC would have been offered a place
b) something actually wrong with the criteria themselves (giving priority in a way they shouldn't, or not giving priority where they must)
c) a decision so unreasonable that it is perverse (in a legal sense)
It's usually hard to win on (c) unless where are very serious matters in play, but it's possible you might have the makings of a case as long as you can show evidence of (roughly) the following: - your DC's needs, and (more importantly) how these were met at school before you moved, - that the appealed for school can and will meet those needs in a similar way, - that current school cannot meet his needs (not just a matter of not doing it in the way you prefer, but really can't do it).
Remember the appealed-for school also makes its case to the appeal panel, so as @BendingSpoons wrote, you need to understand from the SENCO exactly how they would be able to support your DC
And also remember that you could appeal again for next academic year, where ICS rules do not apply, and the "balance of prejudice" can win an appeal (ie showing that the detriment (prejudice) to your DC by not attending is greater than the detriment to the school and its existing pupils by increasing numbers