My son is going through the same thing... he was sick about three weeks ago and he still isn't eating much.
If he doesn't eat what I give him I offer him something I know he will eat. He still LOVES yogurt, apple sauce, cheerios, cheese, crackers, PB&J (as back ups I have some uncrustables in my freezer he adores these - he had them when we were on vacation about a month ago) and all fruit. Also, I recently discovered he really likes ketchup so if he doesn't eat I try to give him some ketchup to dip whatever he is eating in (this only works about 50% of the time). Also sometimes it works if I give him his fruit first then the meal or both of them together.
I try a well balance meal at first but if he won't eat what am I going to do, he is too young to understand this is your only option - when he is older I'll work on that. He is slender so I don't want him to lose weight.
I can't say I really believe the if he isn't hungry he won't eat.
If by the end of the day I know he really hasn't eaten much I give him some extra milk before bed. He usually drinks about 4 oz before bed but I'll pour in 6 to 8 oz if he really didn't eat that day.