Dog Won't Go Play in Yard Where Puppy Pottied?

We rescued a Fox Terrier mix about 6 years ago and he always enjoyed running around and playing in our fenced in back yard. Though he very seldom went to the bathroom in our yard. Last March, we decided to bring home a second similar mix from the same shelter. They get along great and play all the time, however, the second dog is a puppy and has no problem going to the bathroom in our yard.

Now our first dog basically refuses to step off our deck into the yard. We think he does not want to walk in a space his pal uses for a bathroom. I try to be pretty consistent in keeping the yard clear of messes, but the older guy still won't go out there. I would hate that our first dog wouldn't ever play again in his yard. Any suggestions?

By Patrick M.