If you live close to your uncle, offer to walk with him. Set up a regular schedule. Even once a week for 10 minutes is a step in the right direction. Once he sees how easy it is or finds out how much he enjoys being outdoors, then he might continue on his own.
If walking outside isn't possible due to weather or other factors, consider walking indoors at a mall or track. A home treadmill works well for some people. They can watch their favorite TV program while getting some exercise.
Some people are reluctant to exercise. They may be afraid they will hurt themselves. This is called fear-avoidance behavior. Sometimes it's necessary to see a physical therapist and/or a counselor to break out of FAB thought patterns.
In general, exercise gives people a sense of better well-being. But even the most well-intentioned person can fall out of the habit of getting some exercise every day. Having a partner or a supervised exercise program at a fitness center or health club can make the difference.