I am just guessing but it sounds like the first 6 months of his life were hard and made a lasting imprint on his personality. All you can do is just nurture him and give him an environment that promotes socialization, which it sounds like you are doing. Just go slow, as this is definitely not an overnight fix and "pushing" socialization on him could make things worse. As much as you love your babies I am sure this will work out in time.
Good luck!
__________________ Max & Sasha's daddy |