Wow...I'm very sorry this is what happened. Not to be over-the-top here, but it sounds like he was basically abused for a week. That's definitely going to take time to fix. Not impossible to fix but I would brace for a lot of steps backwards during the slow progress forwards.
Have you tried...
That's really good! Don't hold it against him if he backslides a bit every now and then. Training rarely heads in a completely straight line.
I'm also working with a nippy cockerel right now; he's 6 months right now and pretty big, so his bites can be nasty. He has good days and bad days...
Get some leather gloves to protect your hands and introduce the gloves gradually. If your cockerel is biting out of fear due to a bad experience rather than just the usual hormonal angries, I worry a bit that pushing him down as is often done for aggressive roos could make the problem worse...