There's your problem. He's saying no and you're insisting. That won't result in compliance; it will just teach him to say no more forcefully by biting. Instead of insisting he do something he doesn't want to do, wait for a yes (barring emergency situations, obviously). My grey frequently says no in two situations... coming out of her sleep cage when she wants to sleep in a bit longer, and coming out of the bathroom after a shower. I have two ways to get a yes. One is to wait. When she's ready to come out, she'll call me (literally- she imitates a phone ringing, which she learned at some point was a reliable way to get a human to enter the room). That, or I make her want to say yes by making the prospect of coming out more attractive - usually by offering a treat to lure her out.
Some days, I swear she says no just to test and see if I'll respect her wishes. I'll be like, "okay, I'll come back later", and as soon as I start to walk away, she's suddenly ready to cooperate. Other times, she's just happy where she is. She's spent a couple nights in the bathroom. I figure if she's happy and not hurting anything, why not?
Training parrots is paradoxical...The more you respect their desire to back away, the less they'll feel the need to do so.