I'm a inpatient physical therapist and I work with many elderly people that are like this, and it can sure be difficult at times.

First question to consider is, are the exercises appropriate in regards to frequency of exercise sessions and/or sets/reps, intensity of the exercise, time spent per exercise and/or for each session, and type of exercise (as in the general FITT principle). You have to keep in mind that this person is obese, and does not exercise on a regular basis. People as such can certainly follow the kinds of recommendations/programs made for beginners in the wiki section but you have to keep in mind that these are not the only means to an end. Some people may find it overwhelming especially if they are obese and have little experiences exercising.

Secondly, you need to set appropriate goals. Think along the lines of SMART goals. Keep in mind that weightloss is a bad goal because first of all, weightloss is multifactorial, exercises alone is not enough to lose weight (one of the reasons you never have weightloss as a goal in physical therapy). You cannot determine how much weightloss to expect from exercises only without considering the caloric intake. Diet and exercise goals need to be kept separately, since your goal here is to increase motivation (exercise retention) of your client, not to mention that combining the two would also go against the SMART goal setting. Goal setting is a domain that IMO a lot of beginners tend to set incorrectly and end up feeling discouraged.

There are many other specific factors that could be discussed in your case, but lets also consider your goal of "motivating people and making a difference". Looking at it differently, you are trying to impose behavioral change to this person to improve their overall quality of life. This idea may sound like it's overkill, but it won't hurt to know these if you are considering becoming a physical therapist (not that you have to know these though).

There are several theories/models of behavioral changes, including the well known social cognitive theory that looks at the relationship of 3 types of factors that may be influencing a person's behavior, and the transtheoretical model that places a subject in different stages accordingly to the readiness/preparedness to make behavioral changes.

Here is a good summary on behavioral change models that I just googled, there are plenty more to read up by searching for theories/models of behavioral changes if you are interested in these.

Also, it may be a good idea to have him see his PCP regarding his obesity. This may help identify more reasons that he needs to lose weight (which can be used with the behavioral change models).