First, and foremost, you may not be able to force him to exercise. You don’t indicate how long he’s been plagued by spinal pain, but, he may also be dealing with depression from chronic pain. It’s widely accepted that the two can feed off of each other. That, in itself, is enough to blunt any desire to get up off the couch. If you can, try to get a gauge of his mental state.
Early on, I had experienced spinal pain (from a hereditary condition) that was only resolved by surgery. My physical therapist advised me that the quality of my life, going forward, would increase by learning how to manage pain. He indicated the best way to do that is to continue with a daily stretching and flexibility program that I have followed ever since. Your father should benefit from such a program in multiple ways. Beyond the pain management aspect, there’s also the increase in balance and the prevention of falls. As you are probably aware, falls are a serious problem for older, inactive adults. They can lead to serious life threatening conditions.
As others have already stated, don’t expect him to exercise on his own. A good support system is needed to provide motivation and camaraderie. Ask if he would like to start walking. Look at it as an opportunity to share time together. Or, if you have a local community center, bring him for a tour. Community centers typically offer programs targeted at elder citizens.
Lastly, be patient with him. If exercise was not part of his life early on, he’s facing a new challenge. He’ll need your help to face it.