Is it true aggression or just over boisterous puppy play? Any training for him in the near future? Older dogs tend not to like being ambushed by younger dogs that have no manners and the only way that they will get manners is by you teaching them. Good Luck.
It is definitely aggression/dominance. His body language definitely says it too. The first time I thought they were just playing but the puppy latched onto my labradors neck and wouldn't let go. He ALWAYS trys to mount the labrador.
I want to get him in obedience classes but he is too aggressive towards other dogs. I have never had an issue training or controlling a dog but this puppy really has me freaked out. It is upsetting because he is such a great puppy... when there are no other dogs around.