This is just a guess, but I think he is still adjusting to your home and family. Milton bites, but he did from the day I brought him home from the shelter and the people there told me he did. Alex does the "without permission" biting too, but they are really soft and it doesn't hurt at all.
I have also read in tiel books that it is stressful to have a big hand come at them. Evie is a little apprehensive when I take her out of the cage, and she nips, but once again it is really soft. The two tiels I have/had that bit always did from the day I got them. I worked with RB Pretty Boy and she got better, but I really think Bennie is still adjusting. Remember he is still getting used to being away from mom and dad, and I'm sure that's stressful. I think he's still figuring out who he can trust.