Another idea for you if you are sure that he isn't naturally unbalanced or hurting in any way I would try Clinton Anderson's method which is super simple! It worked for my weanling and two big horses. Basically, he says that horses learn from the release of pressure. Here's how:
Start at the knee using your thumb and index finger and slide them down between the tendon and the bone. Gently squeeze with increasing pressure until they shift their weight to the other side. Release the pressure and praise them with petting and high pitched good boys!
(several repititions)
Next, do the same thing but continue applying pressure until they lift the hoof off the ground (Even a half inch). IMMEDIATELY release the pressure to let them know that they did good. Lots and lots of praise.
(several repititions)
Finally, do these steps again and once the hoof is completely off the ground slide your hand around under the hoof and pastern to support it. Do this for increasing periods of time each time. When they begin to get bored holding the leg up, they sometimes will back or rear up as you are describing. If you let go at this point that is saying that they did good because you released the "pressure" of holding it. You need to stay with them if at all possible and follow them backwards, sideways, up or down-wherever they go to get away from you. The SECOND they stand still for you and let that leg just lay in your hand release the pressure and give them LOTS of praise. This reassures them they did good, that running away from the pressure is not the answer and that you are not trying to hurt them. Also, pay attention to how you hold them. For my weanling, he preferred me to just cup the hoof at first instead of holding onto it with my hand closed around it. He felt less closterphobic I think.
Over time, the horse will become better and better at this until they will stand forever on three legs for you. The balance will come. They learn to move out the opposite hind leg for support on their own. To me, it sounds like your little one is just testing you to get you to let go. Let me know how it goes or if you have questions.