After 3 years in your care and at the age of 6-7 years, I would expect the dog's personality to be stable. As in most cases of a new problem behavior, start with a vet exam explaining the problem to the vet. Also, try to think of what other changes may be going on to disrupt his life,
If the vet doesn't turn up anything, I would try obedience training and exerting yourself as top dog. If you are top dog, it is your responsibility to deal with menacing strangers. Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog. Life is much easier if the 2 legged pack members outrank the 4 legged ones. You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at
Raising Your Dog with the Monks of New Skete For more on being top dog, see
Establishing and Keeping Alpha Position