Hi Firstexperience and welcome to the forum!
It sounds like you will be appealing for oversubscription as although your son reached the pass mark he did not get a high enough mark to get a place automatically and hence will need academic evidence in the appeal as well as information as to why it is the right school for him.
There is also a chance from the waiting list - have you contacted them lately to see if it has moved and whether his place has improved?
re the appeal, I think you can just notify them that you are appealing and that the actual evidence can follow later (please see FAQ http://www.elevenplusexams.co.uk/appeals/general
). This should be the academic evidence from SATS / reports and also it would help to get some letter from the school etc. Particularly will need some background as to why he was doing the SATS early - was this an independent or state school?
I suspect the panel will want to know why he did not score higher if he was doing so well at school and I think you may have to discuss what academic work he has been doing this year while you have been away.
Have you checked when the appeals are so that you will be able to be back to attend?