You may not want to hear this idea, but I'll throw it out there anyway.
A lot of people are quick to judge the horse owners who had their horse before them. "He was abused!" "Bad training!" "They ruined him!" It's not to say this can't be true, but as I've found... a lot of horse problems are created by the CURRENT horse owner. Not saying that is the case here! Just something for you to think about.
With that being said, it could just be your horses attitude. My gelding is sweet and kind and tries hard in the saddle, but on the ground he has huge issues with personal space, and can be nippy around certain people. Since you use parelli, you should know that biting means one of two things--a sign of dominance, or fear. So you'll have to look at the situations in which he tries to bite you. Does he try to bite you when you're grooming him? (then it's probably dominance.) Or does he try to bite you when you saddle him up? (Then it's fear, or pain). You can't solve the biting until you figure out what makes him do it.
Ruling out pain, I have a tendency to lean towards dominance. You had the horse when he was young and admitted he was too much for you (been there and done that too... way to go for handling it and not dying! LOL), and I'm sure some of the behavior stayed. Some small part of him thinks he can continue to get one up on you, so he's still nipping! With dominant horses, I know it's much much better to drive away the front of the horse rather then the haunches. Also, you have to start to drive him away /when/ he's starting to bite you. You have to prove to him that you're always going to be one step ahead of him! If he thinks he can bite you... then he can and he will, with or without 'punishment'.
Also, squeezing a horse's lip (like a twitch, but with your fingers-no nails!) after or as he's going to bite you and holding it for 5-10 seconds works on some horses. Just remember to pet the area after grabbing his lip, so he doesn't get head-shy!