While I give you a lot of credit for wanting to expose your child to travel, I would not recommend a Mediterranean cruise for a 5 year old, for the reasons expressed above. My parents took me on my first cruise when I was 7 and we did the Mexican Riviera. The next few cruises were Caribbean. I barely remember those, except for playing with other kids on the ship. When I was 8, they took me on safari in Africa. I remember being sick and tired. I would suggest that you stick to easier itineraries until he is older and can appreciate the historical and cultural destinations.
Crystal Symphony, Crystal Serenity, Seven Seas Voyager, Seven Seas Navigator, Seven Seas Mariner, Maasdam, Norwegian Star, Norwegian Sun, Norwegian Jade, Celebrity Equinox, Celebrity Summit, Celebrity Infinity, Celebrity Reflection, Celebrity Silhouette, Celebrity Solstice, Azamara Journey
Seven Seas Voyager, Iceland/Greenland
Celebrity Constellation, Arabian Sea/India
Silver Muse, Mediterranean
Crystal Mozart, Christmas on the Danube
Azamara Journey, Baltic
Azamara Quest, Australia/NYE in Sydney