Please help roscoe the blue pup anxiety issues

Hello my dog roscoe is a beautiful blue Staffordshire bull terrier he is 1 year and 3. Months old since I have had him there has been 3 occasions people have approached me asked to stroke him and then tried to take him off of my by grabbing his lead either to sell him them selves or to use him for fighting sadly since this has happened roscoe has gone from loving anyone who even looks his way to being very worried and growling and barking at people if they try to approach him outside his hairs on his back do not go up and his tail still wags but he will growl bark and snarl when people walk over to stroke him it is devastating for me because I know how loving he really is and if anyone comes over to visit at the house he is fine just the same loving puppy I raised I feel he has built up an anxiety towards strangers due to these 3 traumatic experiences do you have any advice on how I can help my puppy trust people again he is a beautiful boy and deserves all the attention that he is now scared to receive due to a few horrible people wishing to try and steel him from his loving home and use him for there own gain please help