My Rot isn’t aggressive to any of the stray dogs that approach us when we are on walks. He just stands there and looks at them. Is there anything I can do to make him more aggressive towards them?

You are very lucky, you have a nice dog. Take it from someone who knows. You have a real jewel in a non-dog aggressive animal. Do not reward any sign of aggression. Carry a can of pepper gas and use it on EVERY dog that comes up to you (make sure the wind is blowing in the right direction). I am very serious about this. Pepper gas will teach every stray dog in the neighborhood that you are the biggest bad ass that walks through the valley of death and they all will leave you alone. Trust me on this - it is far better for you to fight these fights for your dog with pepper gas than to try and control a very powerful Rott who decides he hates everything on 4 legs. The first time that one of these dogs attacks your dog things will change forever. Your dog will go from standing there to trying to attack every dog he sees. That then becomes a problem with no end in sight.