A man claims that my dog is really his dog and has tried to steal him.?

Well I know this sounds insane but it's true, I know my dog can't and doesn't belong to anyone else but me. My friend wanted to get me a gift because my grandfather ped away so she got me a 12 week old german shepherd dog. I had him since the day I got him till now. So when I was driving to Machusetts I brought him along for for the ride. When we got to the camp sight a green jeep pulled up behind us I noticed that the same car had been following us since I left from my friends townhouse. A weird old man jumped out and came towards us. He had a gray nappy beard, yellow crooked teeth, long finger nails, and bony hands. He said " You got my dog!" " Give it up!" " He's mine"! I stood shocked and told him he had no evidence that MY dog really belonged to him, his name was Howard and he was apparently 44 years old and I am 21. When night fell I heard my dog barking and Howard was there with ropes and a cage. HELP ME!!! THIS MAN IS CRAZY!