You've already toilet trained your dog so that going outside is routine. Wouldn't it be great if your dog went in one area so that it could be kept up, and you could control the aroma? Create a dog toilet area in a small patch of underused yard space. All you need are a few bags of hardwood mulch, or play sand, and a few pieces of wood that act as separation from the lawn. Anyone with a side or back yard can create a sane, sanitary and convenient toilet area for the family dog.
Identify a convenient area in your side or back yard. It doesn't have to be huge, maybe only 8 by 8 feet or more. If it's grassy, dig down 2 inches and remove the grass. Dig a trench just big enough to fit railroad ties or pressure treated timbers or 4 by 4's at the border adjoining the grass. Cover the area with hardwood mulch 2 or more inches deep with a rake. If you don't like mulch, you can use play sand.
Here's the only hard part. Just as you taught your pooch to go outside, now teach him to go in the dog area. The most efficient way is to wait until he needs to go out. Take him on leash to his new dog area and wait with him. When he takes care of his business, praise him like crazy. Repeat a dozen times. Then repeat without the leash until the idea of the toilet area is fixed in his mind.
Once a week or as often as you like, clean up the obvious evidence. Change the mulch once or twice a year.