My 7 year old dog started peeing in the house whenever he feels like it.

I have a dog door that's always open. For years he's been good about going outside. My other dog pees outside, but my scotty will just pee on the carpet. It started as a weekly mess and now it's daily. We tried puppy pads hoping we could retrain him but he ignores them. I don't know what to do, I'm thinking about building a dog house and keeping him outside. Any advice would be great. Edit: yes we've been to the vet for a joint disorder he takes meds for. I just opened the slider so he can get out without the lip on the door, went to go weld in the garage for an hour and he peed on my boots... He always tries to do it on something too. That and the fact he goes outside whenever he wants makes me think it's not physical.