Laptops with 4k resolution displays seems to be all the rage nowadays. Just bear in mind that the GTX 960m is not powerful enough to play games at that resolution. The GTX 960m should be able to handle most current games at 1080p (or half the resolution of 4k) with high settings or better. Due to the high resolution, your son may have scaling issues from time to time when using programs that have not been designed to scale up the menu and icons when used with a 4k display. This means text and icon will look tiny.
Other than that the Lenovo Y50 is a pretty good gaming laptop.