What I would do (and what I did with my ex bullfighter Horse Don Jamie, who was traumatised by humans for years and then had it a lot better with me but still was used as a 'riding horse' ... So don't feel bad, at least you know now
)
I would let your horse have utter and complete control (As Romy says).
Take his lead a 100% from now on.
If he does not take any lead, just be with him, read a book, or just sit and eat and drink coffee. Talk to him, tell him what has changed in you and why.
Ask him for a second change, but tell him, only if he wants to.
Ask him what he would like you to do.
If he tells you 'just go away' then leave and try just being with him again, the next day.
The other two games, chasing the tiger, and all Brenda and Romy says are a very good idea as well.
But I would wait with any play, until he decides to come to you.
When he does, be overly happy and especially thankful!
Think about what he has been through and still comes to you
Rewards within a second when he comes to you and then start to play.
If he is not interested and leaves, then go back to the simply being with him.
It takes time and patience, but it will be worth it
Think not about result, think about how you can be of service to your horse.
Think about how you can be a pleasure to be with, think about why your horse can be a 100% sure that he can trust you at all times, that you will listen to him.
Think about how much you love your horse, and why he should love you back.
Think about all your horse can teach you, about horses and about yourself.
A lot of it is all in the mind and starts with absolutely doing and wanting nothing.
Keep us posted.