Applying for a statement for high functioning asd - help!!

Hello everybody I'm in the process of compiling my letter to the LA to ask for an assessment on my 10 year old lad. He is higher functioning so his grades aren't way down, my problems lie mainly in his communication and behaviour. He is so easily led, for example he jumped off a building within the school grounds 14 times because some children told him to, he angers at others so easily, headlocks tries to the strangle them etc but manages to "hide" his behaviours from his teachers and letting rip at home instead. I do have schools backing on asking for an assessment but have been told they feel its unlikely because hes a bright boy. Could anybody give me some wisdom on what sort of information to put into this letter or general advise please? I feel so sad that everybody tells me he won't get a statement because he's not failing in his actual progress, why should he not get the support because he masks things and then meltdowns at home instead :( Thanks so much mum25