I NEED DOG HELP PLZZZZZZZZZ?

Ok im watching my friends dog for a month or so but this dog is not really friendly to other people when he sees some one new he will try to Attack them the person will have to move really slow and sit down and talk to the dog before he stops barking and stuff but as soon as they stand up and walk away he trys to Attack he never tryed to Attack me he really like me but not other people he was hit by a car a few years ago and has never been the same BUT ONE BIG PROBLEM I HAVE is my uncle lives with me and every Time he sees him he trys to bite and attack i hold him back and tell him no but there no stoping the dog when he sees my uncle my uncle has tryed givin him treats and sitting down talkng to him but the dog doesent like him for some reason what do i do please help....