There are a few different issues you’ve mentioned here. One thing that helps to keep in mind is to always meet the client where they are–with regard to limitations–when planning their program. If he can’t get down on the floor, then perhaps you could find some chair exercises that will help him with some additional quad/glute strengthening as well as stretching exercises.
For a hamstring stretch (if you know for sure that the issue is tight hamstrings), he could sit at the edge of a chair with one leg straight and simply lean a bit forward. Ensure that although his leg is straight, the knee remains slightly bent to take any strain off the knee.
Hope this helps, and remember at this point in his life you are trying to help him with activities of daily living little by little. If you can get accomplish the goals of a little cardio work, strengthening, and increasing his range of motion through flexibility–he will be that much better off.