I understand what you are dealing with. We had a trip planned last fall when things were getting hot in the Ukraine. My husband was convinced that we were walking into a war zone by including Vienna in our itinerary. He hadn't been to Vienna before so it seemed very scary and foreign to him. There were a few things I did to help him overcome this irrational fear.
I mapped out how far Vienna was from Donetsk where the fighting was and then compared that to a map of home, which he could relate to (i.e. Vienna is as far from Donetsk as our house is from _____). I also referred him to what our government was advising with respect to travel to Vienna. We looked at what the government advisory was (none) and talked about how we shouldn't be concerned if they weren't. I researched and talked through the political situation and what it would mean politically and economically for all the players involved, if the Russia were to choose Vienna as a target.
In your situation, I'd talk about the places you are visiting and compare them to familiar cities at home. Maybe use street view on google maps to show him the streets where the hotels you will be staying, the cruise ports and other places you will visit are so they feel more familiar. Find the airport maps for Amsterdam and Frankfurt. Compare the cities you will cruise past and visit to places at home. Compare casualties in US random shootings and terrorist situations to EU situations and Europe will start to look a lot safer very quickly. Look at who/what has been the target of all the recent terrorist activity in Western countries - it has pretty much been exclusively military, government, counter-Islamic or strong religious groups - it's not likely you will have those types of locations on your itinerary. Would knowing the location of the Embassy in the cities you visit give him some comfort? It would help my husband feel more in control and prepared. Would having the option to rent a car when not on your cruise make him feel more in control? Reassure him that you can wait and cancel your trip at the last minute if you need to so he feels like he has a way out. Talk about how the company operating the cruise has a strong interest in keeping its passengers safe or it will go out of business pretty quickly.
Basically, find as many ways as you can to give him a little perspective and control and to make the places you are visiting feel less foreign to him.
Europe is safe and there is absolutely no reason you should feel any concern about traveling there.