Aggressive Border Collie
I have a 1.5yr old border collie who is the most loving dog to every person he meets. Ever since I got him he has gone to our local dog park almost every day and loves running and playing with other dogs. I have had zero issues with him being around other dogs until the last month; recently whenever there is another dog playing with a puppy, he gets extremely aggressive towards the puppy (charging it, bearing teeth and nipping). This has happened twice in the past month and only with puppies.
I'm worried the behaviour might escalate to other dogs and would appreciate some advise for what to do in terms of correcting the behaviour.