If Cato turns out to be a Rhodesian Ridgeback, not a boxer OR a Great Dane, what do I need to know?

I’ve been having this uncomfortable niggling feeling that he may indeed be a Rhodesian Ridgeback, and that worries me. He’s awfully aggressive about things that are outside of the house, barking loud and low and growling. Just now he went a little crazy at the mail man, and the fur along his spine was standing straight up.
I’m doing my best to train that aggressive reaction out of him, to replace it with sitting or something else, but I’m afraid I may need some professional help here.
He plays with the cat. He gets a little rough with her, and we back him down, but the cat is not afraid.
He seems to be good with the kids.
Not so good with other dogs, though. He was OK as a tiny puppy, when he was around a bunch of dogs at Rick’s cousins, but that was back in July and he hasn’t been around any other dogs since. We went to the lake recently and he about went apeshit when another dog walked past us.

I just don’t know what to think. Don’t know what to do. I love the heck out of him. He’s smart and he’s affectionate.