God bless you for taking this poor little guy into your arms and heart! We will pray for the two of you that he is healthy and gains weight as fast as my skin-and-bones-babies did! I home cooked for them and they just gobbled it up together with the love I put in each bowl...
Oh... he might have some worms so make sure the vet does a stool test. Maybe you can take a stool sample with you... and urine too if you can find something small enough to fit under him...