They should eat less, and workout. Eating processed food it terrible, eating more food would do no good, and just because more vitamins were added to a food doesn’t mean that it makes it better. Usually enriched foods are the most processed.
You should sit down and have a calm and collected talk. start of easy and work your way up to it. Make sure to tell him you love him and that this is who you are. I think he'll understand and he'll love you know matter what.