Sudden aggression in older dog

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
It could be a physical problem, he need to see the vet.  Try waving your hand in front of his face or a ball, and see if his eyes track it.  If not, he may be blind.  See how he reacts to a noise behind him.  If he is losing his sight or hearing, he could be surprised by another dog or a person.  Dogs hate surprises.  Or it could be something else.   

Correcting problems that come up like that with an older dog is not my strongest area.  Unfortunately there is a shortage of good people on the net.  Plenty of know nothings pretending to know much more than they do.  Do a search, and you could find a site put up by a kook with terrible advice.  If the vet can't help, you might try a local dog trainer or behaviorist.  You could also try http://www.k9forums.com/  Some of the people there can be a little prickly, but worth putting up with.