Chances are he's just warning you off and doesn't intend to bite, but there are certainly worse things that could happen to you than getting bitten by a ball python. It's more scary than painful according to everyone who's been bitten by one (luckily, I haven't yet). I'm glad to hear you'll be taking him to the vet. When I take mine, I generally carry them in a critter-keeper type thing with a dark hand towel for them to hide in. A box as you described should be fine, as long as it closes securely.

Oh, and congrats on your new friend!