Oats = good, soreness = not good

So, the oat experiment seems to be going well. Odin gets 1/2 scoop oats with 1 scoop pellets AM & PM, and I have a lot more horse suddenly. He doesn't get tired during my lesson, and is a lot easier to get to go forward. Now, I have to get used to it, he runs a little bit through my hands forward (evading that way instead of slowing to a crawl, I think its progress), but I just think I have to get used to it. Now I suddenly have more power, I need to contain it.

Now the other thing which has come up, some soreness/stiffness. Here the last week or 2 Odin has been a lot more reluctant to canter on the right lead, especially while being straight (or at least a little less crooked). As soon as I try to straighten or do a half-halt he is swapping leads. I can get him a little more supple on the right and it helps, but it is clear that he is uncomfortable cantering to the right.

L. and I talked about it yesterday and today. He doesn't look uneven or stiff/jerky in the trot at all, clear 2 beat trot. In the canter to the right he keeps stiffening his hind legs and swapping leads, especially when first cantering without suppleling. L. ride him today in the first part of my lesson to make sure he felt like that as well for her, she could feel he did the same with her as with me.
L. also poked his back, he was quite sore in his back, in the hind end.

So, what is it?
1) cold weather stiffness? Well he didn't do it in the end of December where it was very cold too
2) the oats? This started at the same time as the oats, and he was on a high-protein feed earlier. The pellets didn't seem to have any difference, but the oats are probably more starchy/sugar?
3) general stiffness. Well he is trained the same, and it does get better with riding, but the sore back indicates that it might be more than that.
4) Hocks. He had his hocks injected about 1 1/2 years ago, and he swapped leads then. It really helped with the stepping the right leg under himself. L. thinks the sore back points to a hock problem. (The saddle fits well)
5) bad riding. We don't ride him that deep/out/over his back, but he is not ridden hollow either, and he doesn't have any glaring conformation issues. I don't think he is ridden perfectly, but I think issues like this would have come up earlier, and I don't think the way he goes when I see L. ride him looks bad.

So, tomorrow the vet was NH coming by the barn anyway, so we will have her look at him, and probably flex him and see if she can see anything. So maybe an injection.

The other thing we can do is put him off the oats, and back on the Purina Ultium, but keep the pellets so he still has more energy. If NH doesn't think there is anything with the hocks we might try that.

He is getting a shot of Legend (HA) every month. I tried Adequan (Glucosamine) this summer and it made no difference, but I might try again, since this is new.

It might also be a combination of all the points, of course...

Well, he is almost 17 years old, and have been during dressage his whole life, so there probably is some wear and tear on the joints, and it is not so that he can't/wont do it, but that doesn't mean he should be uncomfortable being asked to canter right.
Hopefully it will get better.