It seems to be to much focus around food and that could make it more of an issue than it is. If you want him to eat good foods then maybe it might be the time to remove junk foods out of the house for now. Use some of the suggestions Arcadia suggested. If he doesn't want to eat don't force him for now, let him know its ok and he doesn't have to hide it. If he doesn't want to eat it bring it home or leave it on his plate. Ignore the issue at the moment or it could become more of an issue. Then see how that goes after a few weeks. If he is seeming to starve himself more and still hiding food after you have reassured him its ok that he doesn't want to eat it then maybe you might have to look into it further.
And at this point I think the chart would create too much pressure.

Totally get your concerns and as someone who has been through the whole eating disorder thing myself it is worrying to think our children could go through the same.