My black lab is aggressive can anyone help?

I have a 3 year old male black lab. When my family got him when he was a puppy and we didn't know that you have to devote a lot of time to socializing them. The only dog we had at the time was a 12 year old lab who was very dog aggressive. (We didn't get her until she was 2 we don't know what happened early in her life). Never ever people aggressive. Not once in all the time we had her. She got in several dog fights though. She wasn't aggressive towards my lab though. She has since died about 6 months ago and for a while my lab was very, very nervous. I assume it was because she was gone, because she was very dominant and would always let him know what to do. Then after the nervous stage was gone he started to get dog aggressive. It's gotten worse over time and now whenever I take him on a walk he will lunge and try to attack other dogs. If I let the leash go and he doesn't have pulling on his neck he will just sniff noses with the other dog. But if I'm trying to pull him away he will growl and try to bite. I had to get a prong collar to control him. I really want to be able to take him on a walk without being afraid he will hurt another dog or get hurt himself. (He's never shown any people aggression). Can anyone help me?He is neutered and not aggressive towards my cousins dog that he sees maybe once a month (pit bull and a pit/mastiff mix). Also not aggressive towards our bullmastiff.

Do a searh for canine agression, there are lots of resources available.Medically, is he neutered? If not the testosterone is making him aggressive, talk to your vet about neutering.If so, the next best step is to try to train him. Really try to be persistent (Every day/every other day 15 minutes at least). Do simple commands like sit, stay, heel, etc. The key is to get him to a point where you can get his mind of aggression by resorting to a command he knows how to follow through with. It will never be easy, and some dogs just don't snap out of it. But after some time with training, you can try introducing to a friends dog (both of you have leashes on, so that you can instantly break up any potential fights) and work your dogs comfort zone a little at a time.

www.dogbreedinfo.com be pack leader and dont give him the chance to be dog aggressive.