Keyboard + mouse will always be the most accurate way to game and the graphics on PC games are much better, but you need high end hardware to make it better than what you can get out of a console game on a nice HDTV, not to mention the screen is bigger on the TV. Computer gaming was always more comfortable for me, because gaming in a gaming chair was always easier for me than sitting on a couch, but everyone is different.
Does your Grandson like to mess with hardware? Is he old enough, or is there someone else in the family who would want to build him a PC? That way you can spec out a nice gaming system and monitor (if he doesn't already have one) and ensure you get a video card that will run the game at max settings.
He already has a TV sized monitor. Not afraid to build one but my limited knowledge is woefully outdated - like scsi connector outdated. Yes - I am old before my time...