They're spot on with the comments, so I'll only add this tip to help with the shading. You know how you have shaded each fold? Lighter on the parts closest to the camera and darker where the cloth is folded within itself? Well, if you think of the entire robe as one big fold, maybe that will help in the overall shading a bit -- to add depth. I'm not quite sure how else to say it, so I'll just post an example of what I mean. I hope you don't mind, but I did a quick paint over yours. I simply added some shading on the sides of his body, his arms, etc.