I have a very dog-reactive dog I found as a puppy in a pile of garbage (literally) about 3 years ago. He was skinny, covered in fleas, and had a broken tail cut halfway off so I don’t think he had a very good start to life.

He is BFFs with my older dog and does fine if we bring another dog as in like a friend’s dog or something to the house probably 80% of the time. He is also very kind to people, even children, and he loves cats and kittens and is so soft and gentle with them. But walking and being out and about is miserable, he pulls and freaks outs whenever there’s another dog and we can’t take him to bar patios or anything like that, he causes huge scenes and won’t stop barking. He’s not super resource-guardy but the one real fight he ever got in was with a friend’s resource guardy dog and my dog is the one who lost and ended up with a leg bite that needed antibiotics so I think his aggression probably stems more out of warning/protection than a proclivity to actually hurt another dog. He also loses his mind at home any time a person or dog walks by or a neighbor slams their car door or any noise outside he hears, and it works my older dog up because he is definitely the suggestible one and does whatever the reactive dog does.

I can’t figure out what he needs- do I need to lean into his protective instincts and teach him to be like that on command so he feels like he has a job, or do I need to train him to be less protective and relaxed? And either way, where would I start?

It’s probably worth mentioning that idk what breed he is but he looks to be some combination of border collie, pit and chow (blue spot on his tongue), he’s about 50 pounds. Also worth mentioning he got VERY sick shortly after I found him with autoimmune meningitis and had a near-deadly fever and almost died and had to spend five days (and many thousands of dollars) in the hospital and he has always seemed more anxious to me since then. He was on steroids and other meds for almost two years after that and he’s in full recovery now, but that’s his medical history.

Any ideas?