Who does the grocery shopping and food preparation? If it is you, then you can help him by ONLY buying healthy foods, by keeping simple snacks in the front of the fridge so he doesn't have to "hunt" for something to satisfy him.
It isn't your fault, but I am learning that I have to stop enabling my family members by keeping unhealthy, quick and easy , even fast food, foods where they can get to them first. I have to for myself as well as them.
Look up chair exercises on the internet-you will find many you tube videos to show you things he could do to start MOVING his body. As someone said in another post-there is NO EASY WAY OUT-eat less, eat better, move more. I am doing things like walking the dog with my husband because it makes both of us move.