Let me start by discussing an issue that is negative. Nevada is one of the states that has a law allowing adverse possession. Adverse possession is when a neighbor acquired "title" over a portion of your property because they were using it without permission. The time limit in NV varies somewhat, it can be as short as two years or as much as five.
Once the person "gets" adverse possession then the property belongs to them just as if they had purchased it, although they do have to go to court (normally) to get an order stating that it is theirs so they can sell it.
There are a lot if different issues that play into this. For instance, the use must be "open, notorious, and adverse". There is also the issue as to whether the time limit starts over when the property is sold to someone else.
At this point you are going to have to hire a lawyer and move forward. What most lawyers will tell you to do is have the fence torn down and rebuilt on his side of the property line. However, you don't want to do anything in particular until you meet with a lawyer and you get a plan of action into place.
You can find a lawyer to assist you by going to www.lawyers.com and in the section for Area of Practice enter Civil Litigation orReal Estate Litigation. Either of those will have the skill set you need.
If your question has been answered then I'd offer my best wishes to you and ask that you please not forget to leave a Positive Rating so I receive credit for my work.
Of course, please feel free to ask any follow up questions in this thread. I want to be sure that all of your questions are answered. In addition, once you issue your Positive Rating the question will lock open and no longer time out so you can come back to it anytime in the future if you think of any follow ups.