It is WONDERFUL that you are supportive! However, there is nothing you can do if he is not self-motivated. Hopefully your great attitude will help him in that regard. Grain free sounds like an excellent start too, because you can't out-exercise a bad diet.
He needs to move, or the joints will just get worse and the weight, even if it stays the same, will tax his joints further. I recommend he start with a lower impact exercise he enjoys, yes even lower impact than walking. The first thing I ever got my morbidly obese husband to do was bicycle with me. Riding a bike is about as low impact as you can get, and is really fun too.
If you have access to a gym, something else that is easier on the joints for me and great exercise is using a Concept 2 rower - full body workout without the "pounding" of the treadmill or walking/running outside. I also find the elliptical machine to be easier on the joints than the treadmill.
Check out my buddy Ernest and how cycling is working wonders for him:
Instead Of Surgery, Man Pedals Off The Pounds : NPR