I was watching some videos of me riding Jerry, and I noticed that when he trots and canters, he holds his tail out. Not up, but out. Do you think that he's in pain, or is it just how he likes to hold his tail?
My horse holds his tail slightly to his left when we ride. It's normal for him. If your horse just started doing this, I would think he might be a little uncomfortable. Maybe his back would be out? If he's been doing this with his tail for ages, I wouldn't be worried.
I wouldn't if he's been doing it that long. I've seen a lot of horses hold their tails funny and they have nothing wrong. Been checked by a chiro and everything. My horse has been doing it ever since I can remember. For peace of mind, though, you could have him checked by a chiro.