The only way to mute Boomerang is by leaving the enclosure. He is silent when he doesn't see people. Such are the consequences of incorrect feeding and raising. His owners would feed him as soon as he started squealing, without a time pattern or "secrecy". Usually, chicks are fed in such a way that they don't realise humans are the source of food. Otherwise they develop a reflex: squealing and begging for food even when they're not hungry. In the end, this kite grew up imprinted on humans, thinking they are his relatives, parents and lifetime food providers. Unfortunately, a year spent at the rescue centre hasn't changed his behaviour.
Such a bird cannot be released into the wild, or he will come to people, looking for support, and might get killed.