How do I get a formaly abused dog to trust me?

I’ve recently got a 7 month old teacup chihuahua I’ve had him for a week and I’m on the verge of giving him to a shelter he came from my first cousin his children basically used the dog as a toy and as a result it seems to me like the dog is mentally scarred he curls up in a ball in the corner when u aproach him... show more I’ve recently got a 7 month old teacup chihuahua I’ve had him for a week and I’m on the verge of giving him to a shelter he came from my first cousin his children basically used the dog as a toy and as a result it seems to me like the dog is mentally scarred he curls up in a ball in the corner when u aproach him whatever part of his body u touch u can feel the muscles flinching and tensing he got loose yesterday and couldn’t be caught the only reason he came home because it started raining he also bites he shivers when u stroke him and flinches at the slightes movement just doesn’t like human contact at all please help has anyone ever had a dog like this if so how did u gain it’s trust
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