My son is two years old and he responds to the camera sound of mobile phone camera (mobile phones must make a sound here when photographing). I guessed photogenic but that's really right is it? I mean like smiling not really playing around- what is a good word for that response.
This is a knotty one. Of course, there's camera-friendly, which sometimes gets used as an antonym of camera shy, but it has other meanings that turn up more often (like describing something as being ready to be photographed, either because it's always acting in a way that won't produce photos that cause problems, or because the combination of colours works well in photos, or various other possibilities).
Then there's the noun phrase "camera whore", which doesn't quite work either. It's a coarse term, but it refers to someone who makes sure there are lots of photos with them in, takes a lot of selfies etc, and is always trying to look 'good' or interesting in them - and usually that they post them a lot online. Due to the metaphorical use of whore, it's ended up getting a shaded meaning for some, to refer to people who do it in a way that is intended to impress everyone with how good they look.
Some make neologistic use of "photophile", which actually comes from biology and means an organism that thrives in light - but that's mostly used for people who love to take photos (of anything), rather than be in them.
Really, for everyone to understand, you're best with a phrasal description like "he always acts up to the camera".