What do we need to do to maintain ownership of the 3 foot strip of land?

We built a new fence inside the existing fence when we remodeled out yard. We then had a double fence with a 3 foot separation between the fences. The original fence is on the property line and the new fence is 3 feet inside our property. I told the neighbor that the original fence is our common fence and that it needs to stay and is our common ownership. The new fence is on our property and is owned by us. Without our knowledge, the neighbor removed the original fence and now we are concerned that he may try to claim the 3 foot strip of land as his. He is also now using our fence since he removed the original fence. We don't want to move the fence because this is a 1/2 acre property and it would be expensive. Can we force him to put a new fence on the property line?