L.C.
Morning D.,
Honestly the only way anyone will eat what they should be eating is if it is all that is available. Unfortunately, this wasn't nipped in the bud at an early age and now you have this dilemma. Although your son isn't overweight and is very active, that doesn't mean he is not malnourished. Without the proper nutrition, immune systems run down, anemia sets in, and all kinds of other negative effects can ensue.
I truly hope you can put your foot down and not allow those foods. He will get hungry enough that he will eat what you give him. And even if he doesn't eat in front of you...as long as you don't have junk in the house for a few weeks to get him set straight...he may sneak the nutritous food at night while your asleep so that he doesn't starve. Then his body will begin to get used to eating better and you will see that slowly he will do better.
I would definitely make it clear that snacks and eating out are treats and will only be given upon good behavior and eating properly. Good luck.