Canter right looked great at the beginning, then suddenly you appeared to pull his head up...not sure if that is what I saw because of the distance.
Your position could use some tweaking. First just sit in the saddle relaxed, feet out of stirrups.Then take hold of the front of the saddle and pull your pelvis forward and deep, so your thighs hang nearly straight down. Then put your feet in the stirrups, without changing your thighs, and slide your heel under your hips.
Next, lift your shoulders straight up, then relax them while rolling them back and down. (your shoulders are not bad, btw)
This is the correct position to be in.
Try to not wag your whole leg when posting, lower leg should be still, it is only to que not to rock.
So, your horse is cute, and seems to be an honest sort. He is trying to do as you ask, and offers to go deep and round, but you seem to pull his head back up, not sure why? When he offers to stretch down, give a bit with the inside hand and reward with a "good boy".
Overall, your hands were good, the horse is willing but put in awkward position.
The walk to canter transition was not bad, but you are startling him, as opposed to just stepping into canter. This is from the lack of a good half-halt, which should proceed any change of gait.
But, I see a ton of potential in you both. The horse has three good gaits, and you seem to be very much in touch with him.
Are you working with an instructor?
I think what you were seeing was me trying to pick up his inside shoulder at the canter, he gets nice and deep, but will get heavy and dive into the circle, or at least thats what it feels like while sitting on him. I like letting him stretch, but we were going through a phase of him rooting and then throwing his head back up to avoid the contact altogether, so I'm trying to maintain a neutral carriage.
I've been trying to sit on my seat bones, it feels unnatural but thats what one of my friends at the barn (who's more experienced) says I should do. I feel much more natural more rocked forward more onto my pubic bone, but it causes me to tip forward. Which is correct?
He's a very good boy with a fantastic work ethic, he's a bit on the anxious side, so I'm often combating that. I will try half halting before asking for the canter. For a while it seemed like the half halt confused him and made him anxious, like I was telling him stop then go too quickly. No matter how I cue for canter I seem to startle him, I usually say the word canter and will bump him a couple times with my legs to create energy a few strides before cueing for the canter so he's more prepared, but I still get a similar transition.
I work with an instructor as often as I can, which is not nearly as often as I like.