Okay, leave the stock speakers and use the high pass filter on the deck to cut all bass from the stockers, and they will get plenty loud.
Here is a killer loud setup that would work with his amp, you just need to look at the measurements and make sure it will fit in his trunk. It should.
I really recommend holding off on changing stock speakers until you hear it with the stockers high-passed and the sub working.
http://www.ebay.com/itm/CAR-AUDIO-SI...049f97&vxp=mtr
http://www.ebay.com/itm/300882396947...84.m1423.l2649