I'm going to adopt a shelter puppy who'll be 6 weeks by the time I take him. He's my first puppy. He was rescued at 4 weeks, and been in a shelter since then, alone in a cage. I've read it's crucially important to socialize puppies at 5-10 weeks. They also should be spending time with their mother and litter mates. He was deprived of all that. How likely is that he'll be a happy dog if I start socializing him now?

I hope it's not too late.
Problems happen when a dog is orphan and stays alone without knowing anything until a later age. When he is 1-2 years old and meets a dog, he doesn't know how to act like a dog (because he's only been around humans). Socializing at that age is done only with his mother and brothers because it isn't safe to expose the dog to anything outdoors until he has had all his shots. But dogs are still puppies until they are 18 months old and always keep learning, you can start socializing him when he's 2-3 months old (depending on when he finished his shots, if he already did by any chance then great you can start right away) and he will be totally fine. Let him meet dogs and cats and other animals as much as you can (most important is dogs though so he can learn from them but also other animals so he can learn to be friendly and gentle around them) socialize him with children and basically anything and he will be ok.
It is never late to socialze and I'm happy the puppy found an caring and considerate owner such as yourself. Emotion: smile Yes he will be happy dog if you stay by his side!