In addition to what the others have said, you aren't doing very much to make his job easier or understandable.
At the very least, please use groundlines on each side of the fence, rolled out at least a foot from the base. Filler isn't a bad idea either. A single rail with no ground line and no filler is hard for the horse to see and to judge distance.
When I start babies over fences, I like to start out with 3 - 4 cavaletti set for trotting, then a gap, then a little pile of poles. Then I change the little pile of poles to a cross rail and eventually a little vertical.
This helps teach the horse to judge distance, and to bring his hind legs together and engage and push off evenly. (The distance between his hind legs, as well as the uneven knees, really tell you this little guy doesn't have much idea how to do what you're asking.)
Your position - standing up in the stirrups and crotch ahead of the pommel - is weighting his front end and making his job harder. When schooling unbalanced greenies over fences, you have to be very, very conscious of your position and balance because it can affect them so dramatically.
I do have to compliment your release, your hand and arm is relaxed and soft, and there's a nice little float to the rein.
I love this horse's attitude, soft, willing expression and the way he's stretching forward with his head and neck. Figure out how to set him up for success, both by constructing appropriate exercises and by your position, and you'll be on your way to a nice horse.
ETA: I wouldn't be jumping much more than crossrails on a 3 year old.