the operative word here is ‘walker’ and not age. As Paul and Christine suggested, balance, flexibility and strength exercises are very important. Given that he is on a walker, cardiovascular exercises in the traditional sense are not an option but you can help him assume better posture while using the walker.
Since your profile is not completed, it’s difficult to know your background. As Wendy suggests, there is specialty training for exercises for older adults. What you will be able to do with this person depends a lot on his current ability.