I think your ball python is passing on the medium rats in the hope that the more familiar small rats will return. Ball pythons are risk averse, a strategy that works well for them in the wild. Since, he does not need medium rats, I'd let him have what he wants. Start offering small rats every 10 days. Extend it to two weeks of that doesn't work.
Above all, be patient with him. Eating always poses a level of risk for them. Their ability to go long periods without eating allows them to avoid that risk. It's a survival strategy for them. So, by all means, do what you can to make him feel safe, but understand that skipping meals is natural for them. Probably healthy too.
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