I agree with a lot of comments above, he is an adult, portion sizes could factor significantly etc... There is also little to stop him from filling up in other ways if he is not wanting to change his habits which I imagine are quite well established by now.
Does he drink a lot as this could contribute as well and not much you can do to control alcohol intake.
I have to say, I'd lose a fair bit of weight if I moved home. Not that I eat loads but they eat a lot less and in a far more well balanced way than I do.
Good homecooked food might prove a winner though, I know no Sunday roast has come close to my mum's for me.
You could make general changes, skimmed milk, sweetners instead of sugar, losalt (sp?) instead of salt, spraylite instead of oil, leaner meat etc... Healthier ways of cooking things too such as grilling rather than frying, stirfrying in a wok. Maybe low calorie fillers before a meal such as a bowl of vegetable soup.
I found Weightwatchers is quite a good system for learning how much more of somethings you can have to make a meal more filling with less point attracting ingredients.
Being OTT about it might backfire though, I always feel like I'm being watched at home which puts me on edge and is quite upsetting though I haven't lived at home for a long time so visits are easier. As he is an adult it's pretty much up to him if he is to change.