I agree with JLcribber - the very best solution for cage aggression is removing the cage. Birds of all sizes benefit from access to bird rooms or indoor aviary setups.
If you do not have the room for a walk-in aviary or can't dedicate a whole room to the bird, another solution is to switch to a two-cage system. Instead of one cage for sleep, play, and food, provide the bird with a large "play" cage with lots of toys during the day and a smaller more sparsely furnished "sleep" cage at night. In the wild, parrots will roost in a safe place and then travel around their territory during the day to find food and other necessities. Feedings can be done either inside one of the cages as an incentive to switch locations or outside at a designated play stand to encourage the bird to view the entire room as "his" space, not just the two cages.
It might seems counter-intuitive that the solution to cage aggression might be giving the bird even MORE cage ... but expanding your bird's territory allows you to more easily transition him between cages so you can perform necessary maintenence and gives him greater security both in and out of his cage. And it can help him to see the entire room as a safe space and your presence as a normal part of his environment, rather than an unwelcome intrusion.