Hi L.,
Just wanted to share a point of view - by no means the only one you'll get on this issue. I believe you can't make a child (of any age) EAT, SLEEP, or POTTY. You just can't MAKE them do it and, it's not our job as parents/caregivers. Our job is to offer our children healthy choices/boundaries and let them choose (this covers ALL areas of parenting). There are consequences for the wrong choice but you want them to learn to make healthy choices while they are still little (if I don't eat lunch, I don't get a snack later) rather than when they are teenagers or adults when the consequences are much steeper.
Kids respond to attention/reinforcement. So, if he is getting attention and reinforcement (positive or negative) for NOT eating, that is what he is going to continue to do. Unfortunately, it is probably a power struggle that you are losing. Is he going to waste away and die? Probably not.
Also, remember that a child's stomach is the size of a walnut so their serving sizes are sometimes one or two bites of something - really.
As the caregiver, you should gain consent from the parent before you do this but I think you should ignore, ignore, ignore. Offer him his plate and then just make pleasant conversation (non-eating related, like the weather :) )with them while they eat - or don't eat. At a certain point, lunch is done, if he hasn't eaten, it really is his deal, not yours. If he wants a snack because he's hungry later in the day (and it's not a usual snack time) then the answer is no. He may tantrum or whatever (power struggle #2). Or, some parents offer fruit for a snack for non-lunch eaters instead of Teddy Grahams or whatever. He still is young to understand some of this but you are setting the foundation for him living within healthy boundaries.
Also, ask yourself why it bothers you so much that he doesn't eat. Is it you can't stand to throw the food away? Start him out with half the portion then. Is it that he just isn't doing what you want him to? Welcome to the world of parenting! A great question to ask ourselves when dealing with our kids (or relatives for that matter) is Whose problem is this? Are YOU hungry when HE doesn't eat? If not, let him be. It won't hurt him - you are not abusing him.
If his parents are not open to you creating an atmosphere that you both can live with then it is time to stop watching him. Your stress over eating IS felt by the child and I wouldn't want to be dropping my child off somewhere that I knew he was causing problems.
Good luck!!!