Let me clarify a couple of things for you -
Good front end form over fences consists of a couple of different elements -
First, the horse's forearm should be horizontal or above horizontal. Look at the photos of the paint mare and the horse in my avatar photo for examples. Next, it's best if the horse's forelegs are in exactly the same position, not uneven or with one knee below another. Third, the cannon bones should be folded back. Cannons ahead of the vertical is a fault known as "slapping out"; cannons at a right angle are acceptable, the avatar horse's are good and the paint mare's are spectacular. So, to simply - knees way up, even, cannons folded tightly back.
In order to do this; a horse has to have the correct front end confo - well laid back shoulder and a forearm longer than the canon, so he has a lever to snap those forearms on takeoff. The horse also has to arrive at the fence at the proper pace, balance and distance to do so. Good made horses jump in good form no matter what, good training produces that result. Green horses struggle to jump well unless they are helped to the correct pace, balance and distance.
Your green horse is making a typical green horse jump. He's overjumping because he doens't know how to use himself effeciently. He shows some good instincts for jumping, but is not jumping in particularly good form because, well, he's green. He seems to have a nice attitude and is athletic enough that there's no reason why he can't learn. But you will need to help him learn to snap his knees and round up over his fences, and lunging over single fences is not a terribly effective way to do it.
You've been given some great advice by other posters about constructing gymnastics and exercises for him, I just want to add my .02.
I know it's counter intuitive, but a single jump is the *hardest* thing for a horse to jump well. Here's a long quote of mine from another post -
""Three jumps in a line" or gymnastics or grids are MUCH easier for the horse than a single fence. When a gymnastic is set correctly the horse doesn't have to adjust stride or see a distance; the way the exercise is designed sets them up for success and builds confidence.
Conventional training involves lots of simple gymnastics and grids at low heights to teach the horse first, how and where to "wear" their fences safely, and later, to adjust their stride to jump from a difficult of wrong spot. Single fences and courses come much later as their flat work progreses as well.
The normal progresion of training is cavaletti (poles on the ground only), then cavaletti followed by a single fence, cavaletti followed by a simple gymnastic, then more complex gymnastics, then single fences with placement rails to assist the horse, then lines and other related distances, then simple courses all kept fairly low as the horse builds confidence. The horse should also be jumping higher in a gymnastic or something with a fail safe distance than she is a single fence or course. Somewhere between single fences and courses you do a lot of work developing pace, three clear speeds at the canter, and adjusting pace and balance between fences before attempting courses. "
Read more: Nova Jumping her first line of three jumps! Critique