I remember when laser pens first really came out, and I brought one back from abroad.
I used to sit in my bedroom window for hours (I didn't have much else to do), and shine it down on people walking across the busy seafront where I used to live. At that time, not many people had seen them, and it was hilarious watching people go a bit crazy at this red dot shining on their chests.
My Dad took it off me though, after someone knocked on our door and complained!
You can only damage your cats sight if the pointer is shone directly into his eyes.. which you would obviously never do. Just chasing it around, will cause him no harm whatsoever.