I freely admit that I am a bit of a hard-nose about food around here.
I make what I make and you either eat it or you don't, it's your choice. I try to include something fairly kid-friendly and "neutral" at each meal. Even at that, I have kids that won't touch the stuff you would normally think of as a fan-favourite! LoL. I also have a policy that you eat all of your first serving of a meal or snack before seconds of anything else. I found this necessary, as I have a couple of kids that would exist on french fries if I let them. That being said, I will evaluate the food I make. If it seems to be 2 thumbs down all around a few times in a row, I will shelf that idea for lunch/snack for a little while. No sense in wasting food (and my money!).
The way I see it is; really and truly if they are hungry they will eat. I am responsible for one meal a day, 5 days a week (plus snacks, if you want to include them). Mom and dad are responsible for the other 2 during the week and all of the food intake when the kiddos aren't with me! It's really them that will set the expectation. You can do all that you feel like you can, but if McDonalds is a daily staple at home.... hard to beat that.
Maybe if you do feel like he's wasting away, you can offer what you have made for the meal and if he refuses, have a fruit or veggie tray ready for him to choose from? Of course, be prepared to be doling that out to everyone. You can't very well say "no, you eat your whole plate first" but give this little guy his way.