PLEASE HELP!! NEW ADOPTED DOG IS VERY AGGRESSIVE

I adopted a dog 2 days ago, Rotti mix, they think..who seemed like an amazing fit when we fostered him in our Maryland home last week. Well, when he arrived in NJ (our primary residency) Friday night he was a completely different dog. Every dog he sees he locks sights on, he seems to want to play, then BOOM!!! He pounces and tries tearing them to shreds!!! He climbed a chain link fence while on his leash trying to get at a dog. He is solid muscle and made me work to get him down. He exhibits very “alpha” characteristics. We live in a city, where routine walks are part of our daily routine but so is crossing paths with tons of dogs. He is amazing in the house with our kids, he crate trained with ease, and is an ideal family dog. This behavior is horrifing and makes me nervous every single time we go out. We are meeting a behavioralist Sunday but that seems so far away 😧 PLEASE ANY SUGGESTIONS WOULD BE GREATLY APPRECIATED. We are this dogs last hope, because he’s bounced to so many shelters.