Brazilian Rainbow Boas can be nippy as babies, but grow out of it fairly quickly with handling. I prefer to pick up snakes by sliding my hand under its body on the side away from their head in a steady motion. Here is a video showing How To Pick Up A Snake.

If the snake bites you while picking him up, try to continue to hold him until he calms down. This will teach your snake that biting the big bad hand won't make it go away, and he'll learn that handling is a positive experience. I wouldn't worry too much. Most Brazilian Rainbow Boas calm down in time.