If you want to socialize with dogs then only vaccinated dogs that you know.
If you want to socialize to the environment, you can take him places you know are safe. Or if you take him someplace public, try to not put him on the ground.
BUT, you can socialize him to the environment just by introducing him to new things and experiences. Stairs, loud noises, different walking surfaces.
~The strength of one's opinion should not exceed their knowledge on the matter~