Hello Louise
I have had cats with an aggressive streak, too, but I've watched my cats closely and this is something they do amongst themselves...one will wash is companion and show all the signs of gentleness - and then suddenly will bite! But if you're patient with your cat; say 'Ouch!' and then follow up with "Gently, Gently!" in a quiet voice, each time your cat bites you - and keep on showing kindness, eventually it will learn to trust you and (they're intelligent creatures) begin to realise that it doesn't NEED to be aggressive. It may take some time to cure him of this trait, but if you're patient and persistent you'll be rewarded by trust and gentleness.
One of my female cats was VERY aggressive when she was young; she used to hiss, bite, swipe and scratch whenever I went near her, but now she's more gentle than all the others and trusts me totally....as I do her!
Good luck!