Time and patience. Lots of grooming. Start where he is comfortable on his neck, and slowly work your way up. Watch to see when he starts to get uncomfortable, and that is you line. Work up to that line and back down, several times each time you get him out, stopping when he finally relaxes, keeping in mind that you don't want to push him to panic. If he starts getting very worried, move back down to where he is comfortable, and start over again. Over time start working your way closer to his head, small sections at a time, until he's relaxing. Lots of praise, and backing off as soon as he relaxes, and eventually you'll be able to start brushing his face, and touching his ears. Once you get to the ears same thing. A little bit at a time, starting with just touching the base, or the back, or where ever it is that he is comfortable, and working from there. If you do treats, then reward him with a treat when he finally relaxes, otherwise when he relaxes, praise, and back off and work somewhere else. Good luck. I'm in the samish boat with my mare. Not entirely certain what has been done with her, but she really doesn't like my hands being put between her ears. She freaks if I try to slide the halter off over her head, so I am still unbuckling it, and just spending time touching her where she's comfortable, and stretching her comfort zone. It takes time to undo something traumatic, and to reprogram the brain to accepting what you are doing isn't going to hurt. Don't expect miracles in a couple of days.