When I was a kindy teacher I had a student that swore, hit other kids and tried to rip up their assignments. I spanked him and threatened to send him to the "dark room" (closet).
You spanked him?

That is not your right, (only parents have that right up to a certain age), and unless it was before 1976, you broke the law
and abused a child. If you did that now, and the right people found out, you'd be fired, criminally charged and possibly have your teaching license suspended indefinitely. I'm glad you are no longer a teacher.
BTW, IMO I think it was BS for punishing him at school and for making him mis out on the field trip.
The school sometimes has no other choice. Not just to teach the child a lesson about accountability and bad behavior but because a disruptive child can be a real detriment on a field trip. The teacher(s) have to spend more time disciplining and monitoring the child and don't have the time to spend watching the other children. Also, children who are being disruptive are very good at stirring the pot and if their antics caused more kids to follow along, the field trip would be a disaster. What if the child took off? What if others went with him? What if he got them all going and they all started acting up? When children are in a somewhat controlled environment such as a school, it is much easier to deal with them than when they are out en masse on a field trip. It's as much a safety issue for the school as it is a lesson in consequences for your nephew.
You said his parents take away stuff etc.....it obviously isn't working if he's still misbehaving. Maybe losing the privilege, (it's not a right, it's a privilege), of going on a field trip will finally hit home with him and it will stop the undesirable behavior. If nothing else, it is holding him accountable for his actions which a lot more children, (and adults), should learn how to do. One of the things so wrong with today's society is so many of us don't hold ourselves accountable anymore or accept responsibility for our behavior\actions. It's sending children the wrong message IMO, and when they grow up, if you think they have issues now, just wait........