After he was on antibiotics did you treat him with a probiotic to re-establish his good gut flora? If not he may not be getting the full nutrition from what he eats, so he's going to be more hungry.
I would try painting his back half with white vinegar twice daily since he's biting himself as a food response and he hasn't figured out that his back half is connected to his front half. It's not unheard of for snakes to do this - I've seen my king snake and retics grab themselves and wrap, especially at feeding time if they try to grab but miss the prey - but yours is carrying it to an extreme. If his back half smells like not-food and the behavior stops then you have your answer.
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