A new Lesser, Ash HATES him!!!

I've just got a new pet Lesser last night, brought him home and everything goes well. When I just step into my bird's room with it, my sick grey suddenly scream like as if someone is going to murder him! He hates the new comer! Till now he still can't accept it, I don't know what to do. Can any exp breeder/keeper advise me? I have to cover his cage so that he can't see the cockatoo. This have never happen before and I wonder why he react that way, maybe he is jealous? But I ever handle other parrots infront of him and he is cool with it. Why only the new cockatoo? Is it because he is sick so he react that way?