Budgie hand taming / trust help!?

hi, I have a hand tamed budgie (not sure if it was hand fed) that I got from a lady that had him since he was an egg. He is about 2 and a half years old and he will let me pick him up by my finger no problem at all , however when he hops on sometimes hell run his beak up and down my finger vigorously and it looks quite aggressive. He doesn't actually bite my finger but "nibbles" so Why does he do this? And how can I get him to stop doing it? Second He is very very wary about where my hand is at all times. He doesn't like it at all when I put my hand behind him because he always turns around so he can face it. That is also why I can't pet him. Any suggestions? I know this is probably caused by lack of trust but I don't know how to get the trust up higher. I would really like if he would trust me enough that I can lay him on his back! =1 that would be wonderful! So How do I up the trust level and maintain it? What are some exercises I can do? Thanks everyone!