Each client is different, obviously, with regard to their own abilities. Because of his size, I would be concerned and considerate about having him get up and down from the floor. He may or may not be ready for those exercises. I always start conservatively, perhaps with some incline exercises, and progress the client when they are comfortable and confident with their form. In reality, as trainers we should program the exercises around our client abilities–instead of having our clients perform exercises that we want them to do.
That being said, you could try a body weight exercises at different inclines. For example, wall push-ups to start. They could be progressed to a windowsill ledge push-up, to a bench push-up, and eventually to a floor push-up. Some of the same incline techniques also work well with planks. Another great body weight workout option is the TRX system. I use this with many different clients–it works well for all levels. Overall, I would go slow and work with abilities–it will give him more confidence to do exercises he can do well.
Hope this helps you and good luck.