A: Toddlers are notorious for being a little choosy--even downright picky--when it comes to what they will and won't eat. It's perfectly normal for toddlers to be temperamental when it comes to eating and drinking, especially if they are in a new or relatively unfamiliar environment. It's perfectly normal for toddlers, especially when they're not yet able to express themselves verbally, to try to exert control over their environment, which includes what they eat and drink.
That being said, for whatever reason, the child sounds as though his food and beverage preferences there are extremely limited. Offering him water with a splash of juice is a great first step, and it sounds like you really care about the child and his well-being.
My best advice is for you to to let his parents know, in a supportive way, what the situation is and to ask them what healthful foods the child is exposed to and prefers to have at home. Perhaps you can keep at least keep some of the foods you know the child likes on hand to offer him when he's in your care. Offering familiar foods alongside other nutrient-rich options can help him feel more comfortable trying something new. Keeping meal and snack times positive, and offering foods in a relaxed and consistent manner may help the child feel more comfortable to try new, more healthful foods. Offering him naturally sweet fruit and perhaps having a taste test involving all the children can also help the child develop a taste for nature's candy. It may take repeated exposure over several days or weeks before the child accepts or likes new foods, but stay the course. Over time, seeing his peers eating some of the foods he refuses may inspire him to at least try those foods as well.