I've got (((hugs))) but not much advice unfortunately! Dylan is often hit or miss at daycare too, sometimes eating loads but often eating a tiny bit or nothing at all. And his daycare is very good at dealing with his allergies and ensuring that there are at least a couple safe options for every meal and snack for him. There are days that he comes home and stuffs his face like he's not eaten in weeks, and days when he has almost no supper at all. Sometimes I feel like he survived all day on soy milk, 2 bites of banana, and licking the margarine off his bread

I just remember that all I can do is offer safe, nutritious options (at home and daycare) and he will decide whether or not he wants to eat.
The just starting nursery might be a factor too - as he gets more comfortable with the environment he will likely eat more. Also, are they offering pain meds if he's teething?