Cairn Terrier Biting

Question
I adopted a wonderful terrier (Cairn we believe) from a shelter in May.  He is a bundle of joy and has been a great dog.  However, he has bit my boyfriend twice and I just thought it was because he felt "cornered" and was lashing out.  I have a 11 year old son that truly adores "Scout".  Scout has never shown any aggression towards my son, just "play biting" when they are playing.  Tonight however, my son was trying to kill a bug on the wall, it fell on the floor, and Scout walked up.  My son put his hand up so Scout wouldn't step (or eat) the bug, and Scout down right bit his hand - very hard.  It didn't break the skin, but you could definitely see where he had bit him.  I immediately told him "NO", and told him he was a bad boy.  He went onto my bed, and I went to pick him up and put him in his kennel, and he was snarling at me.  I didn't want to get bit, so I left him alone.  I had "dismissed" the boyfriend getting bit because I thought he wasn't talking to him nicely and maybe he felt cornered.  One other time, he was asleep in his kennel, and I was calling out to him to "come to bed".  He didn't come, so I looked in the kennel, and put my hand in and said "come on Scout", and he almost took my hand off.  I just thought maybe he was in a deep sleep, and woke up to see my hand and I scared him.  So I just closed the door and went on to bed.  He has been fine all day, playing with us, and our other dog (they play 24/7 if we let them).  

I don't know the temperament of this type of dog.  Do they normally bite when scared, threatened, intimidated?  Is there any way to break him of this bad habit?  I do not want to get rid of him, I love him dearly - but if he bites my son again, I'm afraid I will have to find another home for him.  Please help me!  (P.S.  My other dog is a black flat hair retriever that doesn't have a mean bone in her body - no matter what you do to her, she would never bite.)  Did I just pick the wrong type of breed?  He was a stray found and taken to the shelter - so there is no history on him.  And yes, I had him neutered after adopting him.

Answer
Hi Linda, Terriers are brilliant and wonderful dogs. He simply does not know that you and your family members are his pack leaders. You must establish yourself as his pack leader. Give him lots of exercise, rules, limitations, boundaries and lastly nurturing. Read up on establishing yourself as the pack leader. Don't worry about feelings, he is a dog and does not have human emotions. If he snips or bites at anyone and the person backs off, he has make up his mind right then and there that he is the pack leader. He must be taught that you will not back down to him, you need to grab him by the scruff of the neck on either side and stare him down until he looks away. This will not hurt him. You can also lay him down on his side and stare at him until he looks away and relaxes his body. These methods do not hurt the dog, but give you leadership in his life. If you are not comfortable doing it, hire a canine behaviorist that is not fearful of aggressive dogs. I handle cases like this and worse every day. So be sure if you do hire someone, that they have no fear, and know how to properly handle aggression. Leave me some feedback, a nomination (if you wish), and please keep me posted on how things go for you. Fear biting and aggression are very serious issues. Please keep me up to date. Thank you for writing. Regards, Susan