Our 3yo German Short Haired Pointer used to get along very well with other dogs in play, however, in the past 6 months he has become aggressive in terms of chasing other dogs down, holding them down and snarling.
If he meets another dog (say on a walk) he will act friendly and then lunge at them and snap.
I can't think of why he has had this sudden change in behaviour, but I am very concerned. We do dominance training so that he knows who is pack leader daily and we do not allow him to call the shots at home; but when he is in this frame of mind he is a different dog and barely registers that we are there. It's hard to train him out of it as I don't want to put other dogs at risk.
Any advice as to what might cause this kind of change and what I can do about it?