Hi all,
This is my first post so please be gentle with me

I’ve read through past threads on dog aggression but don’t seem able to find my specific problem.
My dog in the past month or so has started showing aggression towards other dogs however it is not all dogs and I can’t seem to nail down which type of dog he is going to be aggressive with, male, female big or small this is something he has never done in the past.
Some history: We have had Bode (our border collie) for just over 3 years now and have had him from a pup he is a well behaved dog at home and when out on walks (apart from this aggression problem) and he does as he is told. We used to have another Dog also a border collie but sadly lost him a few month back. The dog that died was attacked around about 5 years ago and since that date always snarled and growled when approached by a bigger dog.
Do you think Bode’s aggression is down to the fact that having lost the other dog he now filling his boots so to speak? Or could it something else?
Whatever it is I would like to nip it in the bud now before it becomes any worse so your help and advice would be greatly appreciated.
Thanks Trevor