House Training an Older Dog?

We have a 9 year old Beagle that was not properly potty trained. But my new girlfriend and I have purchased, a couple of years ago, a poop/pee pad, the type with turf on top and a catch pan under it to catch the urine. We use people pee pads in it. He has done well except occasionally when we are both gone we would come home to pee and poop in different spots.

We have blocked him from that part of the house with kid gates and that helped some. My GF went out of town for about a month and he did not do it that I am aware of. But when she came home he started to pee beside the bed (my side) and we would dry it up and spray the area with urine treatment to kill the odor and deter him from going there again. But he went there 2 - 3 more times then we put down a poopy/pee pad, the people type that are bigger and 1/4 the price. Then he never went there again.

Then 8 days ago the dog had a sore back and we took him to the vet and x-rays showed that he has two vertebrae in his back that were fused together. And now he is taking steroids, muscle relaxers, and antibiotics.

While at the vet we adopted a 10 week old kitten and placed his poop/pee box and food and water on the other side of the kid door barrier. Now he is pushing through it and jumping over it to get to the cat's food. Then he started to poop/pee in the same spot where we had the cat's food after moving it to another area he can not get to. So 1st 2 days were OK, but the last 5 days he has peed and pooped in the same spot where the food was.

We did not try the spray because it did not help with the last time he did this. What should we do to stop him from doing this again? We intend to make it impossible for him to enter that area any more, but are afraid he will continue to do this in the rest of the house. He has also gone where he is supposed to go, but we can not tolerate his behavior.
Please any advice would be appreciated.

By dan Proffitt