I think my Shih-Poo is broken.

Recently got a 8 month old Shih-Poo. He came from a “backyard breeder”. The breeders property was immaculate. Down fall is that the breeder from what I could hear had ALOT of dogs and different breeds on the property. This leads me to believe that my puppy didn’t have much human interaction. This is my 2nd day with the little fella and he’s eaten very little, he can’t go up any stairs or steps, he can hardly walk on his own while attached to a leash. He prefers concrete and when walking on grass he acts as if he’s never seen grass before. Thankfully I have family members who have small dogs so I’ll bring my puppy over to play and has no problems at all. This dog doesn’t act like a 8month old puppy. He doesn’t chew, doesn’t whine, has zero interest in toys and treats. His personality is pretty mysterious and he is super quiet. He’s very snuggly and just wants to be held constantly. How long does it take for a new dog to break out of their shell?