older dog becoming aggressive

Question
I have a nine year old male boxer. He is neutered. About a year and a half ago our female boxer passed away (the two were very close), and about three weeks later we moved. About six months after we moved a neighbor's dog (German Sheperd) got loose from his chain and attacked our dog. Our dog seemed surprised and delayed in defending himself. Prior to this attack our dog had never shown any signs of aggression toward other dogs or people and he even played at a dog park with many other dogs near our old home before we moved. Since the attack our dog has shown aggression toward other dogs when being walked on a leash. Recently (almost a year after the incident with our neighbor's dog) our dog growled and snapped at a person walking by who tried to pet him. This is the first time that he ever acted that way toward a person. Needless to say, I am very concerned. What should I do? I thought of getting another dog to increase his socialization, but now I'm not sure if that's such a good idea. He definitely knows his order in "our pack". He is last after our youngest child and he does not challenge this. He really is a great dog and has always been wonderful with us and our three young children. I know that he's getting older, especially for a boxer. I treasure our daily walks together and could not imagine stopping them. Should he be muzzled during walks? I'm afraid that this will make him more frightened (unable to defend himself). Also, we often chain him on a very long chain in the yard when we are outside doing yardwork. (He loves to be with us.) Occasionaly, a neighborhood dog will be loose and wander into our yard out of curiousity. We usually see the dog before our dog does and we take our guy inside right away. I'm afraid that one day we might not catch it in time and our dog could hurt another dog, or end up getting hurt badly in a fight. Any suggestions? Please help! Thank you for your time.
Maria  

Answer
Hi Maria,
You have some very valid concerns. Please make sure your dog has his rabies vaccine.  If he should bite someone, they will impound him if he does not.  I just wanted to tell you that before I forgot.
About his agressiveness... I may not be the one to ask, you really need an expert on training.  I think the attack by the German Shepherd definitley has something to do with this.  He is also being very protective of his family. Your walks are not only important to you but also to your dog!  He would really miss that time with you.  It may not be a bad idea to muzzle him until you can spend time training him.  Muzzles are not very expensive and do not hurt the dog. Also, I would suggest that you either get some good books on dog training, or find a dog training class to attend.  Many of the YMCA's have these and they are not very expensive.  As I said, you should check with the dog training experts on this also.  I am not very educated on that subject,as I have small dogs and they are wimps......LOL
I wish I could help you more.  Good luck to you.  Boxers are wonderful, smart dogs and I really feel that you can get him out of this bad habit.

God Bless,
Becky