You could always have an extra component available for the weight gainers, like a cup of chili, or bread and butter, half a PJ sandwich, full fat yogurt with fruit and nuts, etc. Maybe a big pile of peas with lots of butter?
OR
You could always have an extra component available for the weight watcher, like a container of crudités, or maybe he starts with a cup of vegetable soup before eating the rest of his meal.
I'm trying to gain weight and eat more vegetables, compared to DH who is picky but also trying to lose weight. I often chop up some extra vegetables and drizzle extra olive oil on my/my growing child's food.That way we get the nutrients and calories we need while DH can eat what he wants without having to pick stuff off.