aggression

Question
I have three spade/neutered shit tzu dogs. Two 13 year old females and a recently acquired 7 year old male. I have had the male for only four months. The owner was excellent but, can no longeer keep the dog so I have him. He was in a home for 6 of his 7 years just with himself as the only pet. When he moved in with us I noticed he began to lay in front of the food dish. One day one of our female dogs went to eat and a fight broke out. The female has always been leader of the pack so to speak in this house. All dogs, visiting and permanent know that she is lead. My husband and I thought he was testing the waters. We decided to put two food dishes down. That only made him on edge. A horrible fight broke out a few days ago. My husband and I had to physically get involved. Yes, I know that was stupid but it saved my oldest dog from serious harm. We feed them separately now. The two females together and the male alone. I heard him growl at the lead dog again tonight. I am afraid their is a dominance problem ahead. I don't want to give him up. I have already grown to love him. What should we do? Are their signs to see that he is dominant aggressive? He gets along great with the other female. She backs down when he growls. At times he gets along with lead dog. They sleep and play together.

Answer
I can't see anything from here and it's extremely abnormal for male domestic dog to not acquiesce to a female, but not uncommon.  The Shih Tzu is a breed that is normally quite docile and accepting of other dogs HOWEVER I have seen rank opportunism (toward other dogs and even toward people) in this breed.  

At age thirteen, the bitch is really not able to hold her "place" in the social hierarchy (in the dog culture) and this newcomer might very well be threatening/testing her.  You need the intervention of a certified applied animal behaviorist (CAAB).  How well socialized this "new" dog is to other dogs is something you don't know and might be playing a role here; his past life experiences are unknown, training, etc.  The evaluation of his temperament and observation of body language among all three as they are in the room together must be done by a professional.  It is not impossible to "fix" this but it might be complicated and I can't teach you how to do it in a text box (especially without being able to SEE all these dogs together).

To find a CAAB see the following sites:
http://certifiedanimalbehaviorist.com/page6.html
http://www.animalbehavior.org/ABSAppliedBehavior/caab-directory

Do this ASAP.  Meanwhile, continue to feed them separately; remove toys and special treats (to prevent resource guarding, you don't want to allow anything to increase this behavior).  Put a house tab (very lightweight leash with handle cut off) on the newcomer (only when you are at home).  If he growls, make no eye contact, say nothing, pick up the leash (within five to ten seconds of the growl) and put him behind a closed door for about 30 seconds.  Do this repeatedly if he growls or exhibits any obvious (to you) behavior that suggests a sudden "attack".  When you bring him back into the room, hold onto the house tab for a few minutes until he is visibly calm, ask for "sit" and praise, let go of the tab.  Do not allow the three dogs to be alone together for any reason without your strict supervision.