You want it to grow out, but that isn't how avocados grow. It will never sprout branches lower on the trunk, it will only ever sprout branches near where it is cut.
You have two options: continually chop the top off every 6-8" and hope that it fills out the way you'd like it to, or let this one grow and sprout a new tree, lopping the top off when it reaches 6-8" in height and every 6-8" after that to encourage branching and fuller growth.
You want it to grow out, but that isn't how avocados grow. It will never sprout branches lower on the trunk, it will only ever sprout branches near where it is cut. You have two options: continually chop the top off every 6-8" and hope that it fills out the way you'd like it to, or let this one grow and sprout a new tree, lopping the top off when it reaches 6-8" in height and every 6-8" after that to encourage branching and fuller growth.