[LisaS]

if it were my loved one I'd start simple - like pick 1 thing to change.
Probably diet will have the most impact for him - so I'd go for that first. You didn't mention mom so does that mean that he has 100% control over what and when he eats?
If he does things for himself, I'd help him figure out his maintenance calories and understand what that means. Then move him into writing down what he eats normally in the course of a day for a two or three days. I say that because it will help him to own the process.

Once you have an idea of what he is eating, you can figure a reasonable daily deficit and work out where to get it in his diet - whether that is changing out cereal for an omelet with fruit or adding veggies to lunch with tuna instead of cheese on the toast or halving the size of the starches at dinner or whatever. Even measuring salad dressing instead of just pouring could make a difference for him.

Try it for a couple of weeks and see what happens. I say this because it works with what he is doing now. Once he is down 20-30 lbs or so he might be willing to make other changes or start more exercise.

Help him to take and record a few measurements too - weight, belly button, chest.