Be careful with rope halters too. They can actually be very severe.
It could be because of lack of riding. Just his way of saying "Sorry lady, I rather not work today!"
How old is he? He could be confused about being ridden in a halter. I know mine thinks halter = graze bridle = work.
Did you watch the owner ride? Maybe he is just objecting to your style of riding (nothing against you) because he is used to something else? You said that when you lowered the reins, he dropped his head. Maybe he is just used to being ridden on a really loose rein?