Behavior Changes in Toy Fox Terrier

Question
We have a 2 1/2 year old Toy Fox Terrier and he used to be fine when anyone came to visit.  Now, if it's not me, my husband or son, he growls and "shows his teeth" like he could attack them.  This has just starting happening the last couple of days.  There have been no changes in the household so we're not sure what his happening with him.  He is not fixed.  Could that have anything to do with his aggression?

Answer
You're very wise to attempt to address this problem at its inception.  Showing "teeth" can be many things, including a fear grimace.  Any display of active aggression toward humans needs immediate expert, in person evaluation.  It's impossible for me to ascertain what your dog is actually demonstrating and to take an in depth history of his life with you.  The internet is not the forum to address a behavior which might result in a dog's failure and ultimate destruction.  contact the veterinary college in your geographical area and find a Ph.D. certified applied animal behaviorist with a great deal of experience in fear aggression and make an appointment.  It's well worth the expense.