What is he hiding?

Yoshio is one of your students, he is smart, nice and helpful. What you notice about him is that he is very talented at painting, he shows you really beautiful pictures and he is always the first to finish. After he is finished he draws in his sketchbook, but when you walk past him and want to look over his shoulder to see what he is drawing, he closes the book very quickly and smiles at you... is he hiding something?