The only thing that I could suggest is take the driftwood out temporarily. Once he does use the cave, he'll always use the cave. It might make a difference if you can camo the pvc by stacking some small rocks around it to make it seem more of a hidey cave. The top of mine is covered with jave fern growing and I planted small crypts around it to make it more of a hideout. Maybe you could also set the pvc into the gravel more, make a spot in the gravel and bring gravel up over the sides like a tunnel. This is how I have one of my other caves. Unless it's a barebottom tank, they like a dark hidey spot. Another idea would be to use an aquarium safe adhesive and roll the pvc in sand or gravel to camo it too, but you still need to make the area around the cave more like a hiding spot. The male wants to perceive no threats around his cave.
