Yep, I agree with Sandy, he's just protecting you from "danger". Casey is bonded to me and one other person in our house, if we're holding her and someone else comes to talk with us, she'll bite me to protect me from the scary person, lol, if they're close enough she'll go after them. Casey was really good with kids when she was a baby but as she got older, she become much less patient with them, lol.
For birds, kids are easy targets because they get a lovely reaction after giving them a good scare, whereas adults are more apt to hold in their tears and screams and reprimand the bird so it's just not as exciting to them. Quakers naturally thrive on drama so hearing your son cry after he hurt him was probably the reaction he was going for unfortunately. It's really difficult to tell a 5 year old kid not to flinch or cry when he gets bitten, which is why it's difficult for a Quaker to have a relationship with such a young child...
Hopefully he can still talk with Kes from a distance and maybe offer treats, but since you're son is so young I'm not sure if other methods would really help.
Good luck!!