hi Michelle
From my work with kids with learning disabiities, I have come to understand that kids with Downs don't have the sensation of feeling full. Is this the case with your son? If so, you need to think of portion control, and some kind of positive activity to signal the end of the meal. What I mean is, plan something he will like doing, immediately after his meal. It would also be worth preparing healthy snacks and or getting him to help make smoothies to satisfy the snacking urge.
There is nothing like family to sabotage efforts, so you need to sit them down, and explain why you are doing this and what they can do to help. you could lay it on a bit thick too about the risk of heart problems, and the benifits of losing weight for this.
i'm using a smaller plate, and it's helpng me!
Sue xx