Just keep to the facts “I noticed yesterday evening his eye was watery and red, did anything happen at nursery? I’ll take him to the GP to have a look as it seems infected” I imagine conjunctivitis is going round, it spreads so easily. They will probably reply and say a number of kids have conjunctivitis at the moment. They don’t usually have to stay away from nursery with it, so unfortunately it does go around. Cooled boiled water on clean cotton pads to clean it, usually goes on its own but they can prescribe chloramphenicol if it’s severe.
Could it be conjunctivitis? And they never noticed especially if he was his usual self and it developed late in the day. But I would say I noticed my son came home with a slightly red eye yesterday, did he have any accidents or did anyone notice this?