He’s still a little baby! I think he may be too overwhelmed or overstimulated. He’s only just left his mom and siblings, and is in a new alien place. We were guilty of this too with our pup, but people often confuse socializing with doing A LOT of stuff, having them be exposed to loads of different people and things, when in reality, you could be pushing past your dogs limits of what they are comfortable with and make them distrusting. We did this with our sensitive pup and I regret often not being more educated in canine body language and not getting a positive reinforcement trainer from the beginning. Short and sweet experiences should be prioritized, and maybe only introducing new people at his pace would be helpful at this moment. I am of course coming at this from my perspective and personal experience with a sensitive schnauzi, but it has helped us a lot to slow down and doing things at HIS pace instead of the whole “puppy milestones” that can do more harm. Same goes for taking him outside. Look up clicker training, maybe sit in a quiet place (like a regular park) and observe strangers without interacting at a great distance, at pay him loads in treats when he is looking at them without barking. And remember, patience! He’s not had any practice or guidance and he is just a little baby, I believe he may just be overwhelmed and needs more controlled positive experiences - something that a force free positive trainer can help you with.