It took some time for Luigi to poop and pee on the same pad as well. He eventually got it, but I ended up buying 2 pads so there are more places for him to go. This seems to work really well. I have pads that I bought from PetCo, where it snaps the pad or newspaper in place so that they don't try shredding them
Just keep at it...he is still young but will get it eventually
Best of luck!