12 month old mini schnauzer aggressive behaviour

(Sydney)

My 12 month old mini schnauzer (Cooper) barks at every chance he gets when walking a leash and sees another dog, even if that dog is across the street. I understand that this is because he feels uncomfortable in some way at seeing the other dog, so I try to distract him by giving treats, but he rarely eats them once he sets off barking.

I thought maybe he wasn't socialised enough (although we did take him out to large dog parks with few dogs as a younger puppy) so I started taking him to the local dog park. He seems to enjoy his time there a lot, although he is evidently cautious of other dogs. He would run up to another dog and play bow, and then run back up to me. Sometimes he would hide under the bench I'm sitting on but once the other dog goes away he's back trying to play. Despite this he has on occasion showed some aggressive behaviour towards other smaller dogs. He would chase them and sometimes I can see him trying to bite. I don't know what play biting looks like and whether that's what he's doing but he only does it to smaller dogs. Sometimes I wonder maybe it's his schnauzer instinct kicking in, as he was originally bred to catch rodents.

Does anyone have any experiences like this? Any advice would be really appreciated:)