Since your client does fall into a specialized population category, it would be helpful to see your profile and background. That being said, medical clearance from your client’s doctor is needed if you don’t already have that. A senior assessment may or may not be necessary, but it’s definitely something to consider. It is important to have a thorough understanding of how someone of an advanced age responds to exercise, and also take into consideration aging joints such as hips, knees, back, and shoulders–and range of motion.
In general, and once you know your client’s medical history and limitations, I would focus on activities of daily living. Many things that we tend to take for granted (such as getting up from a chair or reaching for an object) can be difficult for some seniors. Seated chair exercises may help your client if he has trouble getting around. You can add some range of motion and flexibility exercises to help the joints. You would have to evaluate whether or not your client could do some very light strength training as well.
Go slow with this client, and I hope some of these suggestions help.