I think that with your son, you can tell him that if anything is ever bothering him that he can tell you, and you won't judge him. I have emotionally eaten before and I learned that turning to someone instead of something (food) is much, much more helpful and relieving. Show him ways he can cope like exercise or draw. This is also much better than trying to eat your way out of sadness or stress.
With getting everyone on board with eating more healthily, try making it fun. My husband and I find it fun to try to pick healthier snacks by comparing nutrition labels! We did it today actually! "Hey look, this cereal has only 6 grams of sugar! And the other one has like 13! How crazy!" Tell them what to look for and it will be like a scavenger hunt. Make sure that they also think that the snack looks tasty or sounds tasty.
I hope this helps some.