Why is school refusing to support my child with dyslexia?

my son has gone from 8.2 to 9.5 in reading ability in 2yrs but the school refuse to acknowledge our concern even tho he is becoming increasingly frustrated and his behaviour is deteriorating as a result, we have asked for an education psychological assessment but they refuse. am i within my rights to insist. all they do is bamboozle me and i end up feeling paranoid and weak. what can i do?