Did you ask him what the Latin name was? Might be worth printing off some caresheets and taking them in, sometimes (and I am giving him the benefit of the doubt here) retailers are ripped off by suppliers and if he isn't that experienced then he wouldn't know any different.
The only latin they had on the tank was "gibbiceps" so that would be right. They were bound to know then, exactly what this animal is. I'm emailing all the aquarist societies around me and asking for advice and if anyone would like him. Failing that I will ask the second shop if they can sell him on, it's not like I'm asking them to take in a foot long monster they've little to no chance of getting rid of as they sell young plecs themselves.