You have to tell the school where both boys are involved. That is cyber bullying. And, if it was me...I'd change his email addy so he isn't getting this again.
Do you portion food in your house? We do. Measuring cups and spoons for everything. We have a threshold for junk every day. Once you make a choice...you are done for the day. I'm teaching Nathan to read labels. Instead of sitting with the whole bag of Doritos...count your chips out in a bowl and that is what you get. Still hungry? Have fruit or yogurt. Have a cup of water. You could be thirsty and not hungry.
Nathan is about 5 feet tall and 73 lbs. He is outside a lot (thank heavens) so he isn't sitting playing games on the PC or Wii unless it is early AM or later PM.