question about an adopted dog's behavior

I recently adopted a 3 yr old? terrier mix. He was originally found on Hwy 10 about 2 years ago and the girl that took him in moved and had to get rid of him. He is an excellent dog, very friendly and housebroken. His only problem is that he is very posessive with his toys and when he has them in his mouth you can't go near him. So, I took all of the toys that he brought with him and put them away. I have gotten new things for him, but he acts the same way with these items. He doesn't know how to play fetch or tug. When you throw a ball he sometimes goes after it, but he will not give it to you and growls if you try to take it. Does anyone have any suggestions on how I can break him of this habit? I don't have young children, but am afraid of what might happen when children visit.
Any advice is appreciated.