I need help with training my tiel. He is a 10 month old boy who is very sweet and has never minded being touched or held. He whistles beautiful songs, speaks a few words, and steps up (after some minor biting). However, lately he is biting more and more. I am SO confused. He eats out of my hand. He will sit with me and walk on me without biting me but if he is on or in his cage, he will bite. He has started biting HARD! What can I do to stop this? He is a good baby and I love him desperately but I cannot live with the biting. Please help me!

This is a very hard thing to do, but don't react when he bites. If you pull away, he is winning. When he bites instead of pulling away, gently push your finger forward. I know how hard it is, I've been bitten by a tiel before and they hurt like heck. If you can't do that, teach him to step up on a stick or dowel. He's biting while hes on top of his cage or in his cage because in the past biting has gotten the results he wanted. Teach him to step on the dowel then put your hand up there.
It may take a while but it does work.