Gday Pete
As Willem commented, you are off to a great start here mate.
The one obvious thing to me would be his stance, which is very symmetrical and it appears as though he is tense in the legs? Try to match the stance yourself, and you may find that there is a lot of pressure on the legs and it is not too comfortable after a period of time. His actions would indicate he is at ease, so maybe focus on having the legs 'locked' with the knees tucked under and inwards more, which we naturally do to be more comfortable when standing in the one position for a while. You could then progress to having one hip 'sink' lower than the other and have this leg bent slightly as per your current sculpt, but the other leg slightly behind and more load-bearing, being 'locked' as described above. This adds plenty more visual interest for the fact that his torso can be offset slightly to counter the altered hip alignment and have his body mass/balanced centred. Hope this helped some and I hope you take it as constructive!
Brad