I would suggest liberty it's a great way to have you trust your horse and your horse to trust you! Also Join up( lunging and then after a little while turn your back wait a few seconds and what you hope to see is your horse to come to you when you're not looking if he doesn't that time then keep doing it until he does! This will build trust!
Try the first game of Parelli, the friendly game. Help each other get reconnected! So spend some time with him, groom him, bring him unexpected treats and earn each other's trust back 😊 And give it some time, patience is key💖
It always works great for me to work from the ground every once in a while, whether it's liberty, target training or simply 'hanging out' with them. If you trust each other when you're on the ground, finding that bond when riding will be much easier.