If this is only happening at nursery and its happening every day then there must be something going on at nursery making him anxious. Going by the answer they have given you about not being able to take them to the toilet, i would be very concerned. It is not true, of course they can take them and always should, no child of that age should be left alone in a nursery setting. Often it is hard for children this age to express how they feel about things and something such as wetting when they have been previously dry is a sign something isnt right. I would ask them if you can stay with him for a bit at nursery so you can get a better idea of how the staff are with the children. it could be something that is easily changed if you find out what it is and the staff do change it, but it could be that it is not the right nursery for him and if so it is always best to take the child out and find another one.