There is a chance that it's due to shipping but runny poo and his emaciated condition almost guarentees that he's got some sort of parasite or bacterial infection. So if he passes another lose stool or fails to eat soon take him in for a fecal. Since it sounds like he's already really emaciated, unless this is addressed promptly he probably won't make it.
I've rehabbed starved geckos before, and even though they were just skin and bones they produced solid poos. They also readily / eagerly took food. They usually required hand feeding the first few times. I'd crush the head of a mealworm, squeeze out some guts, offer it on forceps and let them lick it. After a quick lick, they would eagerly take the mealy. The first few feedings I'd only give them 5 mealies at most, otherwise they tend to vomit because they ate too much (I guess their tummies shrank) than I gradually work up until I could give them all they could eat.
Good luck, I really hope you can save the little guy!
Alice