We've had an invisible fence at all 3 of our houses. My dog, who has passed, was on it for most of his 12 years. He started on it at 6m. He only crossed it like 3 times in 12y, and 2 of those was because the battery died.
We have the type of fence where there is a warning signal about 3 feet before the shock line. The dogs learn that when they hear the high pitch to stop or they will get shocked. He was so comfortable with the fence that even when our other dog bit his collar off, he'd stay in the yard because he'd know where the signal line was. On days that we left the yard, he'd be a nervous wreck until we came home. As soon as we put the fence collar back on him, he would visabiliy relax and instantly calm. He was home.
I would say that a 9y old is a bit old to learn about the fence. Also, we learned the hard way not to have a free zone in the front yard. Ours would always race out of the front door as we were trying to leave and then we'd have to spend 15m chasing him around the yard to get him back into the house. At our current house he had roam of the back yard, but for the front, he couldn't go off the front porch or out of the garage.
Is it possible for your family to take him on a good long walk at least once a day?