DS at around the same time got into light and colors. We got him (not all at once) a set of color-mixing paddles, an Alan Baker book, a prism, a magnifying glass, a flashlight, and paints and brushes.
If you want to stay with the doctor theme, add some x-rays and/or a model skeleton, or books like "Your Insides" by Joanna Cole and "Me and My Amazing Body" by Joan Sweeney.