unprovoked biting

Question
I rescued a 2 year shitzu from the shelter.  They said he had nipping problems.  He has bitten me several times now unprovoked and drew blood. He will not let go and goes into an attack mode. Last night he asked to be picked up, I was sitting on the sofa. I picked him up and then he just went crazy. He bite  four of  my fingers, the palm of my hand and my side. He can be very sweet though and I hate to have to return him but my husband and I are both fearful he will really hurts us at some point.  He is very aggressive towards my Wheaton terrier, always barking at her.  She is very gentle and takes a lot from him but will stand over him at time and holding down, does not hurt him though.  He does try and bite her and will growl,eventually stops and she will then get off of him. We had a trainer to the house and he was good with hi, though he is most of the time with us.  The trainer said he had been trained and walked very well on a leash. He is very smart too and understands many words. What to do?  We are expecting our first grandchild in March?

Answer
There may be something physically wrong with this dog....He will probably get more and more aggressive as time goes on.  He could do some serious damage to a baby.  He could have a brain tumor or just be aggressive..This is not a "nipping problem"  I would return him to the shelter.  I am sorry to tell you this...but kindness is not going to help this dog.  And I don't think severe training will help him either...There are so many wonderful dogs out there being put down, why not go get one that is good with babies so you don't have to worry about a bitten hand or even a scarred face.   
I know I sound harsh, but I have seen this before and even when the dog is kept and gets a bit better, it can never be trusted completely...
There are a number of shih tzu rescues out there where you can get a wonderful animal...You might even be able to give him to one in trade for another...They will have more time to work with him and put him in a place where he wont be near any children ever....If one doesnt take him in trade maybe another will.
Nancy