I don't think this is a problem with the horse as much as it is the rider
You just bought the horse. IMO, you're going to have to give this horse some time to bond with you and your son. Friendship works both ways. Just because you want to immediately befriend the horse, he's not sure you're his master yet.
As neither of you know how to ride, the horse is trying you and neither of you know what the horse expects so he's setting the rules.
I'd just keep spending time with the horse, groom him, feed him, hang out with him and ride him in a round pen or small paddock.
Really, I think your expecting to much to fast.
Like everything else, you get out of it what you put into it. Be consistent, pay your dues, and I think the horse will get back to his old self.
For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places.