Well, European External Action Service itself is (according to wikipedia-article is the exact institution to handle such crisis/hostage cituation.
Call me synical, but I have to question why an employee of EU diplomatic corp is spending his holiday on middle east. I don’t it’s unreasonable to assume that EU does some security related intelligence operations in middle east, so to me their employee spending a holiday in Iran sounds like a coded speech for “their work doesn’t fall under diplomatic protection”.
Still, under normal diplomatic relations this still wouldn’t translate to jailing someone for over a year. I don’t believe whatever he was involved in is immoral enough to warrant such.
He travelled to Iran several times before on official EU business, it's not impossible that he fell in love with the country, made friends, etc. and legit just wanted to visit again. Whether that's what happened, and if so, whether it's smart to go there is another question.