Does this sound like torn ACL?

My son and I were walking our dog last night. He tried to jump up onto a stone wall, fell back and yelped and now he's not putting any weight on his rear left leg. He's a little guy, about 14 pounds and two years old. At the moment, he's not expressing an pain, even when I touch the leg, but he's not putting any weight on it. Does this sound like a torn ACL? We're taking him to the vet tomorrow for an evaluation anyhow, but I was hoping to get some experienced opinions. Since he's so small, is it best to just rest him, or is surgery unavoidable? I feel so bad for him - we carried him all the way home. He's a very fit and active dog who loves to walk.