Make haltering fun. Put the lead rope over his neck (gently
). Then, if he allows it, just stand there petting his neck, telling him he is a good boy.
Now hold the halter in front of his head as if your about to slip it over his nose, don't put it on just hold it there. Let him sniff around it. Even if he tosses his head, just hold it there. Next try gently slipping the noseband over his nose. Tell him he's a good boy, he needs to feel like he's doing the right thing. Now if he's comfortable do up the halter and give him a reward.
Remember to take it slowly. He may have had a bad experience with being haltered, so do each step slowly and make him sure you tell him when he's doing right. He needs to associate being haltered with a good thing. If you have the time, tie him up for a bit and just stand with him, so he can get used to being tied up for periods of time. Good luck, and keep us updated with how it goes.