Hello guest. I wouldnt get a puppy just for your 6 year old on because in a few months time he may not want it anymore. You must realise that YOU will be doing most of the work for the dog. If your going to get a dog I suggest that you think about the time and effort that goes in to raising any puppy such as toilet training, general training (sit/stay etc), socialising and expense. Read up on the breed before you get it.
I would suggest you try the Border terrier rescue. Im sure they will give you advise on the breed and you may find your perfect dog or even puppy from them.
Try this website (scroll down to find BORDER TERRIER RESCUES):
http://www.dogpages.org.uk/breedbs.htm