I didn't read the entire thread, but personally, (and I'm sorry if you think this sounds harsh) it sounds like you are just looking for an excuse to get rid of him, haven't you posted him for sale on here before?
As for as the marking and aggression, isn't that kinda your fault? If he was spayed (and you have said you wouldn't use him for breeding because of his size), he probably wouldn't mark or be as aggressive.
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