The best way to encourage the buds along the trunk/branches to grow is to cut them. Its called back budding and works on any part of the the tree. Personally that tree needs to be left alone for a good 2-3 years and let it fatten up. If you want it to fill out you could cut the tips of the branches and it will force some back budding and more branches. Has if ever flowered? If/when it does you should what for any fruit to set, get the size of a marble and then remove it. That will help force the tree to grow itself and not make fruit.

Mike

Thanks Mike, I hope it will fatten up! The new growth so far has just been upward on the top-most branches. It has flowered and gave me three full grown lemons over the winter, I took all the other fruit off. There are small buds forming now, but I'll make sure not to let it set fruit again. I appreciate the feedback!