You have this backwards.
Just lookong for peoples opinions on how to gain his trust and stop the biting.
You need to stop the biting and the disrespect in order to gain his truse. He only trusts (really wrong word here) those that he respects. He holds all others in great disdain.
I would also expect this horse to have other respect issues. He needs to think you are going to eat him alive -- eat him up and spit him out in little pieces. Then, he will respect you and stop acting sour and mad.
They are just like spoiled tantrum throwing little children. Have you ever seen one of those that is happy? They are all mad and miserable and got that way the same way this horse did.
He laid his ears back at someone and they backed up and stopped putting any pressure on him. In other words, HE WON! [You know -- like the tantrum throwing child that Mama lets get what he wants so he will be quiet.]
The next time he laid his ears back and the person did not back off quickly enough for him, so he bit them or threatened to bite them and they backed off.
I am afraid he has been more effective at training people to back off than his handlers have been at training him to have good manners. That is how most horses get mean. Some are just like him. Others get so mean that they are not worth the time, effort or money to fix the problems. [This is how an awful lot of horses end up in the slaughter pens.]
I would groom and saddle him without putting him in cross-ties. I would just hold the end of the lead-rope and I would have a dressage whip stuck in my back jean pocket. The instant he laid his ears back, I would grab the whip and swat him 2 or 3 time across his chest while yelling a sharp "Ah!" at him. Then, back him up sharply (about 50 feet or more). Then lead him back up to where you started and begin grooming and saddleing again. If he lays his ears back again, go after him again only harder this time. [He is not made of china and will not break.] When his ears come up and he starts watching you and keeps them up, just lead him back and start grooming / saddleing him again like nothing happened. DO NOT pet and treat him because he did the right thing. He is not a dog. You will only confuse him. The only reward he needs is for you to NOT punish him. A horse will do anything he is capable of doing for you if you stop putting pressure and negative reiforcement on him. When he does not give you dirty looks, does not push his nose out toward you, does not shake his head at you, does not lay his ears back and he acts happy does not do anything wrong, you know you have gained his respect as HIS herd leader.
It is very important that every horse have good manners; but it is even more important for a huge Warmblood to be well-mannered.