Where to start...
If you prune it hard before it has strong roots, you will not get the results you desire. So first step - make sure the roots are really strong. I would transplant it into an Anderson flat or similar container with really good soil, and give it a season to recover. Give it plenty of room to grow good strong roots - right now that is a small deep pot, when you really want a shallow large pot. Once it is growing like mad in the new pot, you can prune it hard, and the tree will have plenty of strength to push new buds all over the base of the trunk. If you prune it hard when it is weak, you will get only a few buds, or the tree may simply die back or die altogether.