Tossing head at canter transition?

I would try changing bits.

Maybe I can try that, he's had the same bit for 5 years, but that doesn't necessarily mean that it wont help to try and change it. I was thinking maybe a rubber bit because he really doesn't need much to any bit use anyways... Maybe just a metal double jointed as you said though too...

Any updates?

Rode in a regular snaffle bit (not twisted) in a western headstall (only one with regular snaffle I own) and he seemed to like it, but then he got a mystery fever and hasn't been worked so he can recover!
It's always something with horses isn't it?

Very true.. do you think he needs an immune system boost?

But glad to hear that there was a slight improvement when you did work him last :)

Rode him last night and while he wasn't tossing, he's still bracing his head and hollowing his back, which I worked through, but took 20-40 minutes to get a nice headset!

If you trot, changing diagonals every 3-4 strides (on the straight w/o changing direction) what is your horse's reaction? If his head comes up, then there's something off with his muscling and i'd say go back to trot work until changing diagonals every few strides yields no difference with the horse and the movement.

Sometimes even a slight difference in muscling can affect saddle fit or way of going so check that as well!

stop worrying about headset and try trotting on a long rein and see his reaction. If the horse is reaching for contact, then he's loose, if not, keep trotting and bending until he is.

Then try the trotting exercise I just posted. Sounds to me like there is back stiffness and pain.

Oh and forgot to mention if someone hasn't already - get his teeth checked. Even if done recently, a loose, sore, chipped, or rotted tooth can cause all the issues you are experiencing as well! Good luck!

He rides on a loose rein well, and stretches his back when he's allowed to ride with me at the buckle, then will just ride with his head low, at least until all I've said before begins.
The problem seems to be decreasing with time and training, and his teeth were checked and floated after this all started happening, and continued to after it was done