Birds bite for many reasons. A bad diet can make them cranky, fear, frustration, or even health problems can make them mean. And some just come out of the egg cranky.
I have a Cockateil that was that way nearly all his life and nothing I did fixed the problem. But I found that getting him out of his environment MORE will pipe this behavior down. Your bird knows his cage is his domain and will be more cranky inside the cage. But get him out, and he does not feel as territorial.
So get him on a play stand more often, let him sit on the window sill and look outside. Let him sit on a perch away from his cage somewhere. Include him in on breakfast, lunch and or dinner with some cereal, pasta, veggies of some sort or even a seed treat, out of the cage. You have to convince him that you mean him no harm. Make sure he knows the step up command as well. If you have too, use a piece of wood like a perch to get him out of the cage if he is biting too much.
It takes a long time to deal with these types of cranky birds. They can and do get better as time goes on. But most importantly, you need to work with him outside of his cage. In the mental state he is in, he will never come around inside. Get him out as much as possible.
Good luck!