Yes, this is a fairly normal reaction when young students aren't sure what to do, or where to start. It's great that you are trying to practice this with him at home! Try modeling a simple picture, or try drawing one together. For example, you might draw a small rectangle, then show your son by adding two circles on the bottom and a rounded top you can make a car. Demonstrating how to draw and combine simple shapes, that he recognizes, could give him the confidence to do it on his own.