Aggressive cat, not normal bahaviour.

I've had Callum since he was six weeks old. I had three of my own cats when I addopted him. Callum was always used to those cats and then I gained one more of my own after I rescued a kitten who had a lot of health problems, so I decided to keep her. He's always been used to these four cats and got on with them wonderfully. Callum has always been used to new cats and kittens coming into his home for a period of time and then going onto new homes. He never showed ANY aggressive behavior towards them. No hissing, nothing. Just a lovely cat. The only time Callum would ever scratch a little was when I tickled his tummy. Callum never really got used to men as I was single was I had him, this is why I think he is more social with women. About ten months ago when Callum was two years old, I was going through a hard time and he went to stay with my parents along with my three other cats. After a month of so, because of where I was living, I was only able to have one ofmy cats back. Unfortunately, my female cat became pregnant at my parents house, so I had her back as I didn't want her getting pregant again. I met my now fiance a couple of months later, and he became attached to the kittens, so since we were in a big house, we kept them. My intention was always to have my own cats back, but things don't always work out as planned. My parents had six cats of there own (in a large house). Callum has always been used to a lot of cats and never showed any aggression, but when he moved to my parents, something changed in him. For the whole ten months he'd hit out, scratch, bite and attack the other cats. He wouldn't eat with them and even attacked the cats he'd lived with for years. He never wanted to be in the house and chose my Mum as his new 'caregiver'. Only showing her attention and looking to her for comfort. He never attacked people unless other cats were near him and he got confused about who he was hitting out at. This behavior never stopped. The way my parents introduced him to the other cats was to leave him in the cat basket for ten minutes, then open it and let him come out in his own time. One month ago I was in a posistion where I could have Callum back. Since I've had him back, he's shown the same behaviour to my cats and even the one he grew up with. he hits out, growls when they go near him and scratches them. If he's sitting on my knee, he'll attack any cat that dares jump up. We introduced them all slowly. We have glass doors so for two weeks we kept them seperated and they could only see each other through the glass, then we gradually opened the door bit by bit so they could smell each other until the door was fully open and they were living together. I'm at a loss as to what to do and my other cats are scared of him, but he's my baby too. I feel like his behavior is my fault and I should never have let him go in the first place. I did mention that he gained weight after moving to my parents and he never seems to stop eating biscuits. We're trying to restrict his food intake, but he seems to comfort eat.

Just to clarify, Callum is now three years old. He lived with me from six weeks old til he was just over two years old, happy with the other cats. Stayed with my parents for ten months and since then his behaviour has been very aggressive.

Any advice as to how to stop him attacking my other cats would be greatly appreciated. I'm not asking for miracles or for him to start curling up with them and loving them, but just to peacefully co-exist with them.

Thanks