A Camera For Quick Expression Changes

I have had experience in photography for quite a few years. I have scaled down to a Canon Power Shot SD1100 IS Digital Elph about four years ago. Being self-taught, I don't have a lot of background to rely on.

Now...I have a challange...my grandson is a down syndrome/autistic child. His expressions come and go VERY quick. I need a camera that can catch an expression right when I see it because it is gone!! in a sec. My camera lags. I have also thought of continuous shooting. He has more unpleasant face expressions than just relaxed or happy, so I am thinking continuous may not be so good and may be aggitating to him. Could you please help me select a new camera that will be quick to capture what I see?
Thank you.