we have bought a border terrier 4 weeks ago he is only a puppy at 14 weeks old. He is a lovely natured dog and is well behaved but we do have one problem. We have a son who is 9 and our dog gets very over excited with him and chases him and jumps up and nips the back of his legs also he grabs his clothes with his teeth and pulls them until he makes holes in them. My son is getting incresingly upset because he can now not play in the garden without being chased. Earlier our dog got so over excited he bit our son on the finger and made him cry! The vet advised us to squeal very loudly every time he mouths it worked for a short time but now has no effect at all! He is pretty good with us and we also have a 3year old and our dog is good as gold with him! Please can anyone help?

Unfortunately this is a dogs idea of play and he is very young and needs to learn it is not acceptable to us wussy people.

This article should help: http://www.jersey.net/~mountaindog/berner1/bitestop.htm

Squealing can make dogs/breeds with a high prey drive worse.

It is best to teach your son to never does anything to make pup excitable, no running about while pup is around, or excited screaming etc.

When my children were small I fenced the garden in two so that dogs only had access to the part nearest the back door and the kids could play without interference, also visiting kids were kept away from the dogs, and you didn't have to worry about them treading in poo you had missed.

Unfortunatley BT's do get excitable when there loud squelaing or high pitched noises, and as said he is a puppy, and will think of this of a game, you would be best to seperate the pair, and try and get your son not to get too excited around the puppy, and to be calm, and approach the puppy quietly, and give them both there own space, you have really to teach both your son and the puppy how to behave around each other.
I have 4 BT's and know how exictable they can get.

Unfortunately the "Bite Stops Here" page seems to have been removed - links to it don't work any more.

To the OP: at this stage your son will need to play calm games when the puppy is around - if he wants to play vigorous running-about noisy games (as children do) it'd be best if the puppy is kept indoors at the time. There's plenty of time for that when he's a few months older and a bit more sensible.

The article can be found here: http://www.pattesdouces.ca/acces_clients/dog_bite.htm

Here are some others:
http://www.crickethollowfarm.com/biteinhib.htm

Here is the article on another site:http://www.doggonesafe.com/stop%20puppy%20from%20biting.htm

Also here -I haven't checked to see ifb oth are complete, got the impression one was longer than the other?
http://thenewfworld.com/forum/showthread.php?p=70243#post70243

thanks for advice everyone will give these things a go and see how we get on!

Just adding a bit here - I also used 'the bite stops here' what worked for me and mine was the ignoring - it's hard work making sure the kids are trained too but worth it.  My boys have grown up with lots of different dogs [mine + my ex's, mine, my ex's and his new wife's].  Having had puppies in the last 3/4 years they have learned to fold their arms and turn their backs when play with dogs got too excitable and mouthing started. 
I too divided my garden succesfully.

You also need to use a toy to redirect and praise for that...
and, possibly a few minutes time out behind a child gate or similar, but you DO need to be
very consistent with this and stick to it rigidly if you go down this route.
Lots of opportunities for gentle tug games whicn end if pup hurts human skin

See here also:

http://www.apdt.co.uk/dog_training_tips.asp about 2/3 of the way down

Ian Dunbar's article on puppy biting can also be found here

http://www.tristateweimrescue.org/information/info_dog_bite.html

As well as the advice on bite inhibition i would say it is unfair if your son can't play in the garden without being nipped by the dog.
Get some pupy gates, and allow your son to shut the dog in the house or in another room if it is pestering him.