When you do the PS3/PS4 upgrade you get to keep your PS3 disk. What happens on the PS4 is you download the digital copy of Ghosts onto your PS4 hard drive and then you just need the PS3 disk inserted into the PS4 to play it on the PS4. So if you decide to go play on your PS3, you just put your disk back in the PS3 and away you go.
Just know that the PS4 download takes a long time. So plan on it tying up the PS4 for several hours when you do it. Also you will need to redeem the PS3/PS4 code that came with your game if you haven't done so already to get the $10 upgrade. You also might want to change his PS4 button layout to Default Flipped if he's used to the PS3 controls. For some reason they inverted the L1/L2 and R1/R2 buttons on the default PS4 setup. The PS4 graphics will blow your mind.