Check out that he is okay at school does he get on okay with his Teachers and other children, are there any incidents of Bullying there, if he is okay at home the problem is at the school .ask him pertinent questions regarding how he feels about going and why he is reluctant to go. Is it possible you can take a look or sit in at the school.
My husband said he used to vomit everyday before school because of nerves. Maybe your nephews mom could take him to the Dr to see if there is something she can do and make sure that is all it is.