I'm not too great at these but, trimming some of the none essential branches should help the trunk grow a bit - I'd trim the bigger branches in Spring as the plant will take less strain during that time.
If you are doing general, small pruning, prune the tip of the branch as close as possible to the last node, as this will help with forming a new branch there. If you planted it recently, ensure that it stays moist and once properly established, water only when it nears complete dryness.
Looks like a very happy plant though - and nice choice of ground mix.