Facial hair loss

Question
Our Boxer has very little hair growth on his muzzle (particular the really flappy part).  He has a black muzzle but I can still see a lot of his pink skin.  He did have demodectic mange when he was 5.5 months old.  Is it possible that the mange would come back again and so soon?  It does have a slightly vinegary odor (not sure if this is just normal for a boxer) or if it's a result of something he may have.  Any ideas of what it could be?

Answer
I would recommend taking him to your vet since he had a mange when younger.

But, it could be allergies to plastic.

Use stainless steel bowls for food and water. A lot of Boxers are allergic to plastic. Also nylon, so if he wears a collar, or you just put one on him to walk him, use a leather collar or harness.

The vinegar odor could be from an infection setting in... so see your vet for medication.