So we got Stark about 9 months ago.. his mom was a border collie-jack russell mix but seems more collie than jack, his father is totally unknown. Several people have said they see some pitbull in his face. He is nearing 10 months old. We had him neutered 2 months ago. Before the neutering he was starting to mark/pee everywhere outside like every post every tree, anything but never inside the house. Also a couple times before he showed some aggression toward other dogs snapping at them aggressively and going after them. His scruff stands on end frequently around other dogs.
I work from home and am around him all the time, my girlfriend lives with me and a typical day at home is just seeing the 2 of us. Obviously, we get out a little and he interacts with other people and dogs. Usually these interactions go without any issues but recently the problems have been increasing.
We had him at my gf's parents place with a lot of people over and he was really excited and bit a little kid in the back. The kid was on the floor minding his own business, had his back to him, Stark was completely unprovoked.
Then last Saturday we went to a dog park in the middle of the day and it was packed, we let him off his leash and the first dog he was alone with he attacked.. again this morning on leash we passed by another dog and they sniffed for a second then out of nowhere he went after that dog too. The case this morning he had loads of energy and was quite excited to be leaving the house with us, he could tell we were going for a car ride.
So, we have a good idea that he does not play well with others when he is excited. We've also noticed a trend that he acts out more often when my gf has him out solo but we're not sure if that has anything to do with it. I should add that I'm home with him 95% of the time.. he HATES it when he's left home alone but doesn't act out or anything. He just barks and cries as we leave and that probably goes on for 10-15 minutes after we have left.
If I left anything out please let me know, I'd love to hear any thoughts/help on how to teach him that that sort of behavior is unacceptable. He is very smart and learns quickly. We're pretty curious about the father's breed too so if anyone thinks they have an idea I'm going to attach some pics below.. thanks.