Hi there Just a suggestion ,
but have you tried using a blocking technique , When dogs display aggression towards other dogs they normally become focused on this object .
Watch his body language and if you notice a rise in his posture and a fixation in his eyes then this is when you should use a blocking technique stand in front of your dog distract him from the object and get him to sit , he may become agitated and try and Vere around you . stop him from focusing his misplaced energy .by blocking him again make him focus on you .

All this may seem abnormal for him , dogs like people continual learn and change through the years , Also a adamant change in behaviour could mean a health problem and i would suggest a visit to the vets to make sure he is still in tip top condition
Hope this help
Please remember this is only a suggestion
Have a great day The Dog master Lets Go Walkies