Bravo, red, this is exactly what I am seeing here.
I must put in here too. I've been young, and now I'm old, and I was a teacher and counselor in schools during the 70's when pot was legal nowhere and "to party" had just become a verb. The thing I observed through the years after this time was something surprising to me, because at the timeI believed that trying to hound your child into doing the right things, having them arrested if caught with pot, forcing them to live with you when they wanted to live with a more liberal parent or stay with "cool" friend's parents, was going to "drive the child away from you." I believed and counseled people, actually, that the minute that child turned 18 and could choose to get away from this bullying and controlling parent, they would leave, and your relationship with the child would be forever tarnished.
Then the years passed. I watched the children of the more open, liberal parents, those who "understood" and allowed and tried to talk it out through the years. Some of them came out, went on to college, became okay. But a good many also ended up stuck in the drug world, wasting away their lives, becoming addicted to worse things than they'd been doing, etc. A lot of those parents ended up raising grandchildren, or playing "grandchild hostage" through the years with a half assed adult who couldn't be responsible, despite having had great promise in elementary and high school.
And I watched some of those parents who simply wouldn't let it go, who followed up and followed up, and put their kids in private school to get them away from bad friends, and were true hard asses about everything and disciplined the heck out of them every way possible until the very minute they were legally unable to do so. And you know what? MOST of these were the kids who moved forward, did well, and are now happy and successful adults.
OP, the problem is that you handed your child, in today's world, a gold plated excuse to get what he wants, to go for the drama, by the actual physical slapping incident. He flat out admits this to your new wife in his emails. Your protests of being a shadow of your former self since you broke your back, etc. are a clue to an idea that you have, not very well hidden, that if you were the man you used to be, your child would never have back-talked you, you'd have demanded and he'd have given you physical control. But none of this means you do not need to now move forward with what is best for your son, really best for him.
I suggest you do lots with this counselor or another, for your own sense of control and well being, and to help you with future parenting of your now four year old. That sort of thing doesn't fly in parenting, control of yourself and others in your house must come from another place and be established long before a loose lipped 16 year old gets in your face and tells you to "F.... yourself!" or something. And that's got to come from you. Whether you lose this child or not, you've still got a whole life going here with a marriage and another child to move on with. you don't want to be redoing this mess in about a dozen years with another son.
Yes, we all agree, truth is, you messed up there. Yeah, you'd never have backtalked your own father. yeah, you lost your temper and took a swing at him. Yeah, they're going into conniptions talking about what exactly happened, how many times you pounded him, threw him down, choked him, etc. And then of course, there's the truth, and all of you all (as we southerners say) could stand around and camp on this for years. Don't let that happen.
Nothing that happened changes who you are and what you feel is the right thing for you AND your son to do. Your son has known you for many years. You are not going to fool him into believing you're one way when you're not.