Sometimes tray holders discourage pups from going to the pad or getting fully on the pad. You could try using a rubber mat or piece of linoleum under the piddle pad. I use a rubber mat that is larger than the pad, at least 6 inches on each side.
My boys often favor one spot on the pad. It may appear wasteful, but leaving a pad out until most of it is filled would likely discourage your pup from wanting to use the pad. When Max was a puppy, he thought one use was enough to throw it in the garbage.
Keep up the great work of praising your boy when he gets it right. He is still very young, but he will get it eventually. You could try limiting his roaming freedom to be closer to the pad. Then increase his freedom as he learns to go to the pad on his own.