You have really been thrown in at the deep end ann

. Unfortunately you are at the thin end of the governments idea of inclusion
Inclusion does not mean treating the child like everyone else, it should mean giving the child opportunities to access school life/curriculum at their level.
You really need to talk to your SENCO as this child needs some sort of IEP in place. It may just be very small steps at first, but, there are ways to help this child. It would be good to speak to the parents too.
I have worked for many years with a child, both in his mainstream school (when I worked in mainstream) and through my role as a speech and language TA, and he had similar behaviour and diagnosed with GLD. He is now 8 and still severely delayed but is still in mainstream because of empathetic staff who have helped him over the years. he is just beginning to read and write and that is such a huge thing that everyone concerned with him are celebrating.
The big thing to remember is that the child you are working with is probably functioning (mentally and maybe physically) the same as a 1-2 year old would and that is the way you have to treat them to a certain degree. Try and keep everything you do with him short and simple, try and see if there is anything that interests him (although this may be hard) and make sure you get to see a full speech and language report as this will give you some sort of idea of the age level he is at.
Sorry for the big rant

This is an issue very close to my heart. I do believe children should be given all opportunities to have a full education, the problem is most mainstream schools just haven't got the resources to deal with these children and yet children with learning difficulties can make up about 10% of an average 30 child class

I know that the majority of schools do try their best, but it is usually the TAs that shoulder the burden and do all that good work (often in their own time too)
Special schools are excellent, I work as part of one even though my focus is children in mainstream schools, but special schools are so few and far between now, it is increasingly difficult to get a place
Good luck with it all, I will hunt out some websites for you to check out