it looks to me like you are still getting to know his feel and his rythm. he might be stiff, but he's got a good consistent rythm in the trot, and seem to have a good work ethic. I think he will mature very nicely, as you will become more stable in time, too.
I feel like your stability is a bit iffy at times. it looks a bit like you lean forward, or fall forward some times. and since you have your hands angled downward, you are pullin down on the rein when he is reacting by going up . this sort of position, where the elbows become locked in too open of a position, the thumbs are rolled down and there is a downward orientation and rolled forward shoulder and body over the core , and you often looking down, too . . . all lead to the horse hollowing out more, and reacting defensively to the downward pull of the rein.
I know that in parts of the video you were riding on a very loose rein, so I'm not trying to say that you are being harsh on his mouth. but, rather that it seems you are tryin to get him to keep his head down by putting your hands down to counter that. He will only counter THAT by raising his own.
I would suggest that you radically raise your hands, and maintain a supportive contact, especially on his outside rein. or, give him a super loose rein and put your own hands on either you stomach, or on his withers so that you can resist the temptation to pull them downward.
also, would you feel ok/better with stirrups down one hole?