As the disc space collapses the nerve root exiting the level (in this case the L5 nerve) can get irritated and radiate pain down the leg. He does not have a recurrent disc herniation so a discectomy alone will not provide him any relief.
Because there is a lot of disc space collapse and the exiting nerve root is affected, an anterior approach to "jack open" the disc space would be best. A posterior approach for the surgery would not allow as much disc space distraction. Fusing the disc space would not only help relieve pressure on the nerve, but also would fuse the severely degenerated disc space and help relieve his chronic low back pain. With the interbody fusion either a cage or bone dowels could be used to keep the disc space distracted while it is fusing (the surgeon will choose the method he prefers).
Of course, the patient's other option would be to continue to live with his discomfort and try to manage his pain as best as possible with conservative treatment. He is otherwise neurologically intact and is not in any danger if he does not have the surgery.