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The most likely thing in a ferret of that age and with the improvement that you have seen would be parasitic- that doesn't just mean worms, its also very common for ferrets to be affected by a protozoal organism called 'giardia' which causes all these symptoms, and would have been very effectively treated with the antibiotic and worming combination that he was given.
I'm glad he is better. If it reoccurs then i would seriously consider sending a sample away for fecal analysis as that would confirm giardia if it is present. I strongly suspect that is what he suffered from.