My Dog Wets His Bed And Sleeps In It (when He Has The Whole Kitchen To Do This In). How Can I Get Him To Stop?

I have a papillon just under 2 years old. He’s been neutered. He’s recently developed the habit of wetting the bed he shares with my maltese and refusing to sleep in his own. Then he actually lays in it afterward.. I don’t keep them crated when I’m not around. I simply block off the kitchen with a baby gate and let them play there with the training pads down. When we first got him, he’d religiously use the pads, but now he’s decided to use the bed instead, and now ignores the pads. The vet had us switch him to a lower protein and lower fat dog food (called Solid Gold) which also has cranberry in it to support kidney function. They were unable to find anything serious other than slightly elevated kidney activity, but he said to take him off puppy food which we did.
Does anyone have any idea why he’s doing this and how to make him stop? Also the first bed we’ve given him he refuses to sleep in anymore and he doesn’t go near it and as far as we know has never peed in it. He only wants to sleep in Kenzie’s (our maltese) bed with or without her in it… which is the bed he’s been wetting in! I am so confused..