1. It all depends on your honest appraisal of yourself, in terms of ability to re-school a horse like this.
1.1 That includes a huge amount of gut instinct to recognize when the horse is trying to please vs. when it's trying to suck you in
2. I'd go right back to the beginning with him. Pretend he doesn't know anything, including what a saddle is for. I'd stay off his back until his ground manners are back in proper working order.
2.1 Horses like people have different levels of learning and comprehension - figure that out first.
2.2 Make his initial lessons short with plenty of reward (I am death against food treats in this type of situation. Plenty of scratchies, high neck massage, verbal praise.
2.2.1 The minute he "gets" something and does it, stop right there and praise him. You always want to stop with you being the winner
3. Move on to something else using the same method but I still wouldn't make the formal schooling portion longer than 30 or so minutes. Spend the last part of your time just being with him, playing/brushing. Everything is a lesson good or bad.
When I used to re-school rank horses, time was never money to me. It took me longer to get a horse "done" than professional trainers but I didn't care - it's why I never took horses on for money. I refuse to be on a clock when it comes to working with horses.
That being said, once the horse knows the rules and is well under way, I have been known to tell one of them "I have until midnight if you don't do this because you know better"
Best of luck, I hope you end up with a great horse