Biting Baby Senegal

We just purchased a baby Senegal parrot a month ago. He just turned 4 months old. He gets along fantastically great with me. He gets along pretty good with my husband. Until about two weeks ago, he got along pretty
good with my 9 year old son. Then without ANY provocation - I supervise ALL activities - (he was perched on my finger first time it happened) he became extremely aggressive towards my son. He bit my son and drew blood. My son did not react (having been warned beforehand that birds love drama). The parrot will hiss, crouch and snap and try to bite my son as soon as he sees my son come anywhere near him. There is no mistaking the intent of the bird. My son is one of those kids that is very gentle with animals and is very mature for his age, so I know this is not contributing to the problem. The weird thing is that I can pick him up in mid-hiss and he will immediately calm down and start bobbing his head up and down, extending his wing, and being totally happy with me. Any suggestions? Thanks!

I am having a similar problem with my recently acquired 2 1/2 year old Meyers. Very friendly with me but hisses and tries to bite my wife when she approaches. I am hoping to work through this over time. Good luck to you and your son!