Recently, he expressed his frustration about his teacher and classmates labeling bitcoin as a ponzi scheme or a scam each time he talks about it.
Sometimes that's also true - but not all of it.
There are several ponzi schemes that pretend to be bitcoin as an investment - while investors are promised huge returns within a certain time frame. The teacher wasn't wrong - it's just that he generalized it all the same, it had to be changed.
I'm seeking a variety of advice that I can offer to my nephew in this situation. Although his father suggested that he should stop discussing bitcoin, I don't feel comfortable with that approach. What are your thoughts on this matter?
I would side with his father and ask him not to talk about bitcoin to those who hate bitcoin. You don't need a softer approach on someone who doesn't have the right perception of bitcoin - so leave them alone and move on with your plan.