tossing his head

Hello!

The horse I exercise for some people, Chip, has been tossing his head when I ask him to turn left. It seems to be something that pains him, but may just be stubbornness, with turning left, and I was wondering what you all thought it could be. He moves his neck to the left fine, and he'll do a HALF circle to the left but as soon as it comes to closing the circle he tosses his head and goes sharply right. Could a horse do this if his teeth need to be done? His legs look good.
He has also dropped his head, pulling on the bit HARD (knocking me off balance when riding him english) while we're canter/loping. Not sure if that's related or not.
I don't know what else to add.. ask questions.. heehee any help is appreciated.

What type of bit are you using?
It could definitely be a sign that the teeth need to be done, and I'd look into when he last got it done.

Are you asking different during that time? How does your tack fit?

We're using a D ring snaffle on him, both western and english (they don't have two different bridles... they don't know much about horses at all.) Tack fits fine. It seems to have gotten worse, too, beause at first he'd just refuse and then do it after a few attempts when he learned he sorta wasn't going to get out of it. That was probably a month ago. Now he just flat out refuses by throwing his head up and jerking his whole body right. It's mainly when we're going in circles or crossing the ring and need to make a semi-sharp left.
I am not heavy on his mouth, probably too loose actually at times cause he's used to them being in more contact, and with my western pleasure background I feel strange holding his head too much. He'll pull his head around when we're standing still and I give a light tug, which makes me think it can't be neck pain... but I just don't know. I've never seen a horse do it before. I always ask him to turn using light pressure from the rein though, because his sides are EXTREMELY sensitive to pressure and he will canter off really fast if you give much leg pressure to him at all.... which I learned the hard way the first time I rode him! Haha

Bump...

I also asked when he last had his teeth done and they said it's been about 18 months. He's 11 and lives in a pasture (with no grass). I rode him last night and asked him to turn with a lot less rein and it got better, but he still was being weird about it. Has anyone experienced this!?