You need to find out how well he can move his own weight around, first, before having him lift anything. In fact, his own body weight will be the best workout he can do at this point I'd wager.
Fat man pushups (progress from near vertical downwards till horizontal)
Fat man pullups (same deal)
Deep knee bends (or as deep as he can go and get deeper over time)
Walking on an inclined treadmill(glutes and hams and much more difficult than you might think, esp. for overweight folks, but easy on the joints)
45 degree hyper-ex (hams and low back)
Seated db presses and then standing later as reps improve
planks on knees and elbows for time - progress to elbows and feet, then hands above head and feet.
Wouldn't be bad as a circuit. Start low reps and sets and build up as his conditioning improves.
He should always be trying to increase the range of movement where possible, making the exercise harder, faster,longer or heavier where appropriate. And logging his results each session will enable to see his progression over time, which is encouraging and positive feedback.
When he gets through 5 circuits of 15 reps on each, it may be time to do some weightlifting.
Just an idea.