I'll try him in a different pad and see if that makes a difference. I don't think it's a mouth issue, he just got his annual shots a month or so ago, and I believe his owner had his teeth checked also but I'll ask her to see. And I feel like I'm using lots of leg with him, but maybe not!
After watching it again I can see how it looks like he might be trying to stretch but can't. I don't think it's the saddle since it's what I've always ridden him in, but maybe so? Maybe it's a little too far forward and is pinching?