Help, my puppy follows everybody?

I have a 5 month old male shih tzu and he follows everybody and anybody, especially strangers. It's giving me a headache, I have to go out to get him multiple times a day :( We've had him for almost 3 months now, so he knows who his family is and where his home is. Like even when we're in our front yard, and some stranger walks by on the sidewalk, he runs after them and jumps all over them. Today he ran all the way across the street to our neighbor's house and we had to go over there to get him. Then he followed this old man to our mailbox (our mailbox is together with several others) and almost followed him home! I couldn't get to him, he just ran all over the place. Thank goodness we don't live on the main street and not a lot of cars come by, but still. He's only been acting like this for the past week and a half. I don't think anyone did anything to him, everything else is normal. Someone please help T_T