This is excellent base modelling. I would lengthen his "base trunk" a bit, so he is taller over all, and add a few more branches as well. Perhaps move the face up a bit on the trunk also. To add a bit more realism, you might consider making him asymetrical. Trees do not generally grow with symetrical sides like humans do. A lovely bark texture, and some leaves will give away what people are seeing to them. People need visual clues or they do not know what they see. I cannot wait to see more of him, Treants are amongst my favorite people!