Sudden aggression in 5 year old boxer

Question
Hello Jannie,

We have a dogwalking business in Sydney, Australia. Typically we walk 8 dogs at a time off leash in a great big park where there are other dogwalkers. We have never had an incident of aggression as we have become a very tight pack. Two of us have the business but I am largely pack leader. A desexed 5 year old boxer that we have been walking since a pup is pretty much the alpha dog of the pack. In the past, dogs from other packs have approached him in an aggressive manner and he has just walked away. The past two Fridays, which I have taken off, the dog has attacked a larger dog (malamute then Great Dane) on each occasion, totally unprovoked. Apparently both were very scary. We'll be talking to the owners about it but it is difficult to know what to suggest. We plan on walking him on lead to settle but he is so big and strong that we will be looking at an hour of frustration and slight pain every day. Any suggestions you have will be greatly apprec iated.
Thanks,
Lee

Answer
Dogs do not see size. So a larger dog has nothing to do with the attack.

His behavior is becoming dominate of the territory- like the alpha of the dog park.

The only way to control it is on a lead at all times and reprimand when he misbehaves.

The owners need to walk him and experience it first hand to keep him on a lead to control his behavior themselves. When he becomes aggressive, turn him away from the other dog and get his attention on the person with the lead.