How long have you had the child in care? Has he always been picky?
If he is gaining weight he is getting enough nutrients. If he isn't getting them from you then someone is feeding him something that he likes.
FEW child will actually starve themselves and few would push their being picky enough to the point of passing out due to low blood sugars. If he is seeing a nutritionist then they would have noted if he was one of these few pretty quickly in their assessments.
I know it sounds really mean...but if he wants to not eat to the point of passing out...then perhaps he needs to pass out? So you can call an ambulance and get the parents and medical professionals attention.
I suspect he is a strong willed child who is getting food on the side (cheerios from you and likely some filling foods at home). Progress isn't made by sneaking food to a child. It is VERY hard as an adult to see a child go hungry because we as adults know how uncomfortable it is to be hungry but that child needs to feel it himself, to the point of caring to not choose to feel those hunger pains.
I am saddened at how we now have protein drinks for kids and formula marketed just for picky kids. It is a sign we as a whole are not having proper expectations for children's eating habits. We've catered to them to the point in not expecting them to eat real food. It's not good!
Best of luck with the little guy...hopefully home fully jumps on board, it would only take a few days (at most) of a VERY hungry child for the child to broaden his "liked" foods.
Everytime my almost 5yr old goes through a slightly "picky" phase I go into a no easy safe foods, only 'weird' foods cooking streak. She doesn't continue to rely on the familiar common foods and just naturally forgets to be picky as she isn't getting bits of 'safe' foods to hold her along. Meaning no breads, crackers, pasta etc. It seems to work BUT we've has this mentality from day one so it is MUCH easier to get back on track when needed than starting from scratch years into picky eating habits!