Working With a Nervous Frightened Dog?

I have a 6 month old Jack Russell who I bought from Gumtree 4 weeks ago. He is very nervous and growls at people when they come into my house. He is eventually coming around to me and my husband, but not my daughters and son. When they try to stroke him he wees a large amount and cowers on the floor or if we are sittting down he will jump onto our knee and not move. I don't know if he has been abused or if he is just a nervous dog. Is there anywhere I can take him to get him some help or a link on the internet I could look at. Thank you.