I don't own a cat, but I understand from those who do own a cat and a dog, that the dog will often accept the family cat, but still won't tolerate other cats

Regarding the aggression, was he castrated recently or some time ago?
From what you have said, there seems to be no obvious reason for it (ie no recent fights with a male dog which may have got him defensive) so I would suggest a visit to the vet, as any fairly sudden change of behaviour in a dog which cannnot be explained may be some sort of health problem.
best wishes
Lindsay