You may need to check your diplo status with him to see what's driving him crazy. Different leaders seem to get crazy over different things. For some, it's only sometimes about border issues (such as Elizabeth, who will hate you if you settle anywhere within 20 tiles of her because she thinks all land is hers), but others could care less- such as the Pacal, who you can build right on top of and he won't mind, but if you take a wonder or CS ally he wants he'll lose it.
Alexander is is one of those who is really aggressive based on army size: he wants to pick a fight with someone, and if you look the easiest, it'll be you he hates.