growling & barking at certain dogs

Hi, we have a 1 year old cockapoo and the last few weeks he has started to bark and growl and certain dogs we see on walks. Some dogs he will just walk by focussed on treats or look at them but not be fussed, but others he will bark and growl and is fixated on the dog.
He can be quite anxious and had a few bad experiences with off lead dogs when he was a young pup, but will happily play with some dogs.
Has anyone else had this problem? I’m trying to keep him focussed on treats when we see dogs approach but it’s when the other dogs appear more suddenly that he starts with his “aggressive” barking and growl.