It's a bit difficult to animate sprites of this scale and adding in-between frames is worse.
How many frames do you want to add? How is he like (in terms of personality)?
Little sprites usually move and do something in place instead of breathing. I'm not saying to scrap everything (it can be helpful in other cases), but try to think about the idle in a different way.
If you want to keep him breathing or stand still, go crazy with the hair and the two blue stripes (think how the wind could hit them).
If you want him ready to fight, you could think about another pose, more suited to fighting and go from there.