How old is he? It is quite common for dogs to change temperaments as they age until their personalities are mature at 3 years old. With somebody the right age in the family, 4-H dog training is a great idea. In my area, clubs form soon after the first of the year. That seems a long to let the problem fester, but realistically, the holidays are almost upon us and a lot of what else will be done this year is already done. Even many urban areas have 4-H. For info look in your phone book under government listings for extension or cooperative extension offices. Ask specifically about a dog or canine club. The dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog. Life is much easier if the 2 legged pack members outrank the 4 legged ones. You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands a treat. Start at
http://www.dogsbestfriend.com/. If your son or daughter isn't old enough for 4-H, perhaps you can attend a class or work out of a good book.
In the mean time, start applying these rules to him,
http://www.dogbreedinfo.com./topdogrules.htm
Note, almost always, if a behavior problem crops up, start with a vet visit. Since it isn't your dog, you might not be free to neuter it.