While BRBs are considered an "intermediate" species I see no reason why you could not properly care for one.

First off, like others said humidity is key with rainbows and especially with juvies. I used coconut fiber bedding (comes in a brick) and its excellent at holding humidity and resisting mold. Depending on how big he is you could go get a plastic tub and poke air holes in it, your terrarium might be too large for him for now. Also, try adding some damp terrarium moss to parts of the enclosure. Make sure its enough for him to completely hide in, between the moss and the coconut bedding he should feel nice and secure.
Like others said, let him be for a 5 days to a week and then try to feed him. when I got my first one I fed outside of the tank but after that I fed mine inside the tank. if you have a picky eater you might have more luck feeding inside the tank. And if the temps are out of whack that could be why he is refusing food. Rainbows do not tolerate high heat as well as other snakes so be careful not to let that happen.
and about getting nipped, welcome to owning a baby rainbow boa!
most are nippy as juvies but should calm down.
Good luck!