IMHO letting him sort his own dinner is a 'no no'. From personal experience it's too easy not to stick to eating right and cheating when eating separately.
Normally high carb foods are things to steer clear of. Pasta, bread, cheese and fried potatoes (chips or crisps etc).
Have you looked into Slimming World Extra Easy plan? It's not a hash diet and is very easy to stick too. You don't shed lots of weight overnight, it's a gradual thing. It's also designed for those wanting to maintain a healthy weight by adjusting portions, so great for the whole family.
Has he always been overweight? If everyone eats the same family meal maybe he is snacking at work? Does he eat at work or take a packed lunch? Could be that the problem isn't eating at home