Put a pole on the ground in the middle of the arena.
Start cantering over it. You want the horse to take a step right in front of it and then hop over the pole, not just take a stride over the pole. Practice, practice, practice. This will help you develop your "eye" for the distance. You'll start to be able to see "Oh, I need to ask him to collect a little more" or "Oops, we're 5 strides out on a short one so I need to ask him to lengthen so we can get there in an easy 4."
Also, work on the horse's adjustability and strength. Do lots of flatwork and hill work to start helping him build muscle and gain strength. It'll be much easier for him to jump from the correct distance when he's strong enough to do so. As for adjustability? You want to be able to have a five stride line and do it in 8 strides, or 9 strides (or 10, or however many you think you can possibly get without stopping or breaking to a trot), then come around and do it in 4 (I'm not saying do this right now, but this is what you need to be working towards in the long run). In each gait practice asking him to shorten his stride (without pulling on him!) and lengthen his stride (without letting him get off balance). This is where the conditioning and strength of the horse comes in. He can't be balanced and do all these things if he's not strong enough.
Also, get a knowledgeable trainer. Sooner, rather than later.