he wants atleast 60 fps in casual games at 1080pGTX 1060 3GB without nay doubt. On what screen resolution is your son going to play Games, 1080p ? Even the 6GB GTX 1060 would be an option, but if he is just a casual gamer, then you might consider some other option.
okayThen, just grab the 6GB variant, IF there is not a very HUGE price difference between the 3GB and 6GB models.