It sounds like he may still be a pup, and puppies do all sorts of things for fun! He may be a little bored and this is exciting - let's play with the pee pad.
Just be lucky he is not tearing it to shreds - I have come home to 3 pee pads torn into confetti - not fun to clean up. So what I did was bought Doggie Litter Boxes and I placed to wee wee pad inside the box.
Some wee wee pads have tape strips on the back side, so you can temporarily stick them in place.
Best of Luck
