Puppy socialization & health

Question
I recently brought home a boxer pup who is now 9.5 weeks old. He's up to date on all vaccinations, but I'm concerned about introducing him to an environment where other dogs have been (such as putting him on the floor at the vet's, the pet store, sidewalks at our apartment complex where other people walk their dogs). I'm not sure if his young age makes him more susceptible to catching diseases other dogs may have left (such as parvo). What age is safe to start socializing him with other dogs and introducing him to new situations/environments without worrying about him catching something?

I should also note that he was diagnosed with generalized demodectic mange and is being treated for it. Should I worry about an immune system compromise that could make him more susceptible to catching something?

Answer
At 9.5 weeks old, I definitely think he is too you to be playing on the floor at the pet store, and I would also be very careful at your apartment complex...you have no idea how many dogs have been around there, and if they are vaccinated or not.  The biggest fear is parvo...it is an extremely contagious, very hardy virus.  At 9.5 weeks, your puppy's immune system may still not be working to its full potential.  There is no telling exactly when the mother's antibodies wear off and the puppy's own immune system takes over, so I would recommend getting a least a couple rounds of shots before letting him explore too much.  The floor at your vet's office, while still a risk, I believe is less of one.  They know when an animal is contagious, and should clean wherever one has been.  Also, the cleaning solutions they use on their floor are sure to kill any virus your puppy might get.  Check with your own vet, but as a general rule, puppies get 3-4 distemper/parvo combo vaccines 2-3 weeks apart as they age.  This is because, like I mentioned, we don't know when their immune system actually takes over.  I would wait a few more weeks at least.
   To answer your last question, demodex can be a sign of an immune system that is a little suppressed.  Demodex is passed from the mother to the puppies, and normally their immune system can handle the parasite, and it never manifests.  However, sometimes it can be too much for their immune system to handle - that's when you get the hair loss and signs of mange.