Horses that haven't been properly schooled are often one sided. Horses are often more compliant on the left side (right side in the brain). The off side is the reaction side. Start by asking him to bend his head in both directions. Don't force it. One his near side he may bring his nose close to his elbow yet on the off side you may get only an inch before you get resistance. That's ok. Be patient and work at this until you get at least a noticeable bend. Sometimes a treat under his nose to guide him helps, but just one. When he is supple enough to bring that nose around, and it may take a few sessions or more, you have just removed a lot of resistance. Before you do anything with him, spend the time and do this first, as a refresher exercise. You will find that sometimes the offside is more relaxed than the near, it switches back and forth. It also helps set the horse's mood to a more relaxed one. Let us know how it goes. He will be more sensitive to a knotted halter, not a flat one.