I suggest you don't show him negative numbers, but rather, encourage him to invent them. Confirm to him that yes, there are numbers less than zero (you've already done this!) and ask him how they go. Explore together the consequences of what he decides. At this age he isn't going to get confused about something he's about to be taught in school; it's in some ways even more interesting if he invents a variant that doesn't work quite the same way as the conventional negative numbers than if he discovers the conventional ones. I remember my son inventing three dimensional shapes with their own properties and quizzing me on them - lots of fun and great for mathematical thinking!
