Is he sleeping on your bed, when this happens? He may be afraid to get down because of the other dog. They do make doggie depends. Also, if you are home make sure he goes out every 2 hours. If you can, let him out alone, so he can take care of business without the other dog bothering him. Treat him like a puppy, with lots of "good boys" after he comes in. If you have a doggie door maybe going thur it causes some kind of pain, after all he is older and it may hurt his joints. Just a few things I could think of