R-E-S-P-E-C-T

Getting forward movement and impulsion can be really difficult in horses that have no go. Often when round penning at liberty these type horses that don't want to go will look for ways out, even challenging the fence line.
I would try two different things with a horse that tries to escape forward movement. We will start with the easiest and both techniques are on the ground, not in the saddle. The goal is to re-establish forward movement, which means you are going to have to be very, very cue specific. These ground cues are going to be the same cues you use once in the saddle.

Put him in the round pen. Point in the direction you want him to go, cluck and move him away at a walk, as soon as he walks off release all pressure. If he stops, point, cluck and apply pressure until he moves off at a walk.
Do this exercise slowly and only work on walk, when he walks off with light pressure and keeps going than go to trot. It could take days at a walk. Do the trot the same way you did the walk. As soon as he is going the speed you want at a trot release pressure. In the beginning, it is best to go slow with a horse that wants to jump, he is just scared. Once he can truck around at a walk and trot with the cluck cue, work on canter. Going from a trot to canter is easier, so point,cluck and apply pressure until he's trotting around nicely, then point, kiss and apply pressure until he canters. He may be really unsettled at a canter at first because of his age and he may do all kinds of things with his legs, that is ok. He will get settled the more he goes around. The second technique is that if he tries to jump out again and your in fear of him getting hurt, than put him on a lunge line in the round pen. If he tries to jump out than you turn his head and shoulder towards you.
The cues are the same, cluck for walk and trot and kiss for canter.

He will eventually get it, I promise. Even if your already riding, I would redo the go cues on this horse. Once your back in the saddle, have someone in the pen with you, someone that can help encourage forward movement without killing you, I'm not kidding there. Someone who has a clue about pressure and how little is more. I stay completely out of the horses mouth when asking for a canter, I do it in this order. Get your trot with a squeeze and cluck then squeeze with your legs again and use his relearned kiss cue and spank him on the shoulder with a crop or metcate reins. So squeeze, kiss and spank. Keep kissing and apply pressure with your tools until he goes into a canter. If he will not canter and only trots super fast, then ask your friend in the round pen with you to apply pressure with a kiss and crop to encourage forward movement. That part is actually called flagging. So back to basics I would go, learning the cues again. Round-penning, lunging and flagging. If those three don't work, than maybe he is just not mature enough yet to retain his lessons. One thing worth mentioning and it is just my opinion, as I really am not sure what is going on with your horse since I did not actually work with him, but I have learned in the past that the last thing to put on a horse without go is a whoa. As long as you stay behind his shoulders and don't cut his drive line off, than he should maintain forward movement. I wish you the best of luck, forward movement is the hardest to get on a horse that has no go.. hopefully other members will share their experience and knowledge to help you get back on track too.

Wow -- I really like these comments. I have nothing more to add here other than to reiterate I'm all about the ground work and to stay safe. I like the idea of having someone in the pen with Penelope when she has Oliver under saddle.

Kind of a fun note - we're working on an upgrade right now that will let us put videos right into our profiles. It would be cool to see Oliver on video ... and to see video suggestions of techniques to try. Can't wait. I think I just spilled the beans!

And the "edit" of individual posts is something I think is an excellent idea. We're trying to get it to where you can only edit your own posts and not be edited by others! You wouldn't think that would be hard, but it kinda is!

Keep the comments coming guys!

Yes, I agree with Chase and gentlehorse too. Always rule out physical.
I would like to also add that discomfort comes in a lot of forms to horses. It could be fear, stubbornness, lazy, lack of confidence, age, etc..
I have been riding horses for over 25 years and I have never seen a youngster not move because of feed unless they were extremely maul nourished. But you learn something new all the time, so thank you gentlehorse. Also and I'm not trying to argue, honestly, but it is not that uncommon for a horse to try and jump out of a round-pen. I could be as simple as your using to much pressure, or he just does not know what you want so he tries to escape..you could also put one of your other youngsters in the round pen with him that already knows how to round pen and let the young horse kinda follow. Actually, now that I think about it, I probably would try that first. Then do him separately.

I agree with ground work and once you establish enough trust, and that you are the boss, then maybe you could give him a challenge. Start jumping him on small cross rails and see how he reacts and if he enjoys it. Its one thing to not allow a horse to do something (like jumping a 6ft fence) but its a whole other thing when u ask him to jump something that you choose.... maybe he will hate it and keep his feet on the ground for now on or maybe that's where his heart is. So next time u have him on the lunge line, put some rails up and see how he reacts....

Rockin' idea billygrl -- super smart!