if he's jumping on the fore, i'm betting he's not properly engaged or muscled on the flat either. First and foremost it sounds to me like he is not at ALL ready to be shown OF. Next, i'd go back to some basics. I'm going to sound like a broken record here, but there's a reason why so many people preach trot sets - because they work! Soooo trot, trot and more trot. Lots of serpentines, changes of bend and direction, circles, figure 8s etc., staring on a long rein to allow your horse to stretch down and reach for the bit without you holding his head up or asking for a "frame".
As he gets more fit you will find your horse seeking a rein contact - at that point you can take a light following soft contact on the reins. Please remember - reins are not for steering or brakes, they are your telephone line to communicate with your horse! Your primary steering and brakes come from your seat and leg.
That said, once you can trot consistently with all of the above without the horse losing engagement of hind / back when you change bend or direction, then you can progress to jumping. You can start jumping a little sooner but i'm hesitant to push a horse OF before they are moving properly on the flat as I find it tends to encourage them to jump from the fore, even through gymnastics.
One of my favorite exercises for horses is to set up 3 jumps one stride apart each with a ground pole in between them to perfectly set the striding. The jumps should be x-rails set no higher than 18" in the center. Format is x - stride - x - stride - x with striding marked with the ground pole. Take the horse both directions over this exercise until the horse is able to jump smoothly and calmly without rushing or getting "stuck" and you can simply go into your two point and stay there without having to guide the horse. The goal here is for you to do as little as possible while staying centered and out of the horse's way and focus on YOUR position - heels down, back flat, shoulders back, eyes up, etc. let the horse focus on HIS position - knees up, engage hind, balance, steady pace, etc.
Once you have mastered this in both directions, make the ground poles small verticals no more than 2' in height. You should start with them at 18" and after mastering it at that height can increase to 2'. This would make the combination look like this - x - vert - x - vert - x - essentially an all-bounce line. Because it is low, it will not be that technically challenging for the horse but the fact that there are no strides between the fences will encourage use of hind end and back end. And by setting distances accurately and precisely, the horse won't have to worry about where to take off or land, the combination will do that for him. Again you focus on you, your horse can focus on him, with the goal being to ride this as smoothly as possible in both directions. It's a great exercise for both horse and rider, and the key is to not interfere!!!!! Let your horse figure it out. Because the fences are so low, it's okay to let your horse knock rails the first few times - soon enough he'll figure out the combination and learn how to navigate it without relying on you for balance or support!
Excellant post CJ -
I just want to add, that you have to be a functional rider, to help your horse be functional at his job. You must be consistant as a rider throughout your whole ride - to the fence, over the fence, landing and depart from the fence.
When you are on approach to the fence, just as CJ stated, you must be balanced over your horses center of gravity, and over your feet. You cannot be on your horses forehand at all - you must rock back to rock your horse back under you.
When you are going around in your two point, or even full seat - regardless of what you use - every upstride of your horses gait, you must use your legs to say "come up to me" lifting your horses back up into your seat. Even when you are taking off and in mid air over the fence.
Our horses reflect 100% of what we are doing in the tack, or not doing - they show our holes. So if your horse is going around on his forehand, what is it that you are not doing, or doing, while in the saddle?