I can tell you from my own experience that my boy did not take to the grass pads or the potty pads.
The grass pad he liked as a bed and the potty pads he did not tell
the difference between those and carpet.
So i got him the rascal dog litterbox.
Wow , he is doing awesome.
He is 6 months and i never thought i would make any progress till i found this box. It uses a grate system instead of grass or pads.
Check it out....
You can buy the Big Squirt minus the grass which is what i got...
http://www.rascaldoglitterbox.com/