I adopted a 4-5yr old Lab from the SPCA about 4 weeks ago. He is very good and listens well. He seems to be a very fast learner. But he seems to get more and more scared of men each week. I walk him daily on trails at a local park (between 1-2hr walks). His first week he was very good. He just ignored people and other dogs. His second week he started getting unsure of men. He wouldn't walk by them and he would give a little bark. He has progressed to barking and lunging at men. And he will not walk around other dogs anymore. He has had no bad experiences with me at all. I am very confused with this behavior. He is fine with kids and women. It is just men. Any advice how to fix this would be greatly appreciated.