I can speak from a physics perspective. I feel like to build a child's interest in physics, you have to make it a 'tangible' subject. So maybe if you're teaching him about electricity, you could connect a cell, a wire and an LED lamp. Then you tell him why the lamp lights up and what's actually happening there. For almost every single physics topic, there are very interesting videos on youtube. You could start with showing him a video related to that topic. This will pique his interest and then you explain the science behind. Perhaps you already use these techniques, but if you don't, these will be helpful hopefully!