ZaZu Yellow Nape Amazon Parrot impersonates a horse whiney

This is my adopted 6 y/o yellow nape amazon parrot (ZaZu). He was previously housed in a small cage and never let out for 3 years. I have only had him for 8 weeks now and he steps up on my hand & shoulder and allows me to scratch his head. He only whiney like a horse and only does this when excited/scared. He hates my son for unknown reason, probably other owners had a kid that looks like him and did something bad to ZaZu.