Maybe he just doesn't like your MIL. Jazzy has certain people that, for reasons known only to herself, she CANNOT STAND to have in our home, and others that she LOVES LOVES LOVES–gets very excited when they come over, tail-wagging, etc.
Maybe have your MIL become a font of treats. When she comes over, give her treats to give to Jackpot. Every time she approaches him, there's a cookie involved! He may well learn that he LOVES GRANNY because she's walking cookie jar. Eventually you will be able to give a treat every other time, then every third time she approaches, etc. until he finally just accepts her.
He may get used to cars in time. Probably they're just something new and noisy. And he's very small -- I think they'd be intimidating if they are close.
Jazzy was well-socialized: by the breeder, and we took her to puppy classes, walks in parks where there were other dogs, etc. and she still hates dogs.
Sometimes that is just who they are.