Sara,
What a fun way to celebrate! If he likes characters, DEFINITELY take advantage of the meets. There are SO MANY! (And if you do, the plan to buy the photo package.) The lines are longest on the at sea day. We also took pictures with everyone. Our room attendant and a bride who got married on CC. Everyone signed the autograph book, too!
Does he enjoy fireworks? If you are worried about too much noise, ask at guest relations for the best place to view from a distance. (It is typically a bit breezy for fireworks, so the sounds aren't so bad.)
That being said, check with guest relations on the ship and ask them for ideas! They want to make your magic happen. There are so many activities. Maybe some of the cooking and craft opportunities for the whole family? (I don't have a navigator handy. We only just decided to do this cruise last night.)
The Midship Detective Agency can be played throughout the trip. I think it is a lot of fun! I love when the art comes to life, and I bet he will, too....even if you aren't doing the MDA, so many of the art pieces move!
Are you participating in the Fish Extender trade? Even if you aren't, it is fun to put one up, and one of you can sneak back and put treats you know he'd enjoy in it. Who doesn't love coming back to the room and finding a special something waiting?! If you are participating, take him to deliver treats if you think he'd like it. It is a fun way to see all of the door decorations. SO MUCH CREATIVITY! You can do it a deck at a time. Again, if you aren't participating, maybe go grab a bag or two of easter candy being cleared out (be sure it is clearly labeled for allergens,) or maybe some temporary tattoos? Something small and easy to carry. It's a nice bit of exercise.
If you think he'd like to dress up, dress up for pirate night! Lots of adults do.
Tell us a little bit about him. Favorite characters or snacks? I'd love to help make his trip a little more special, and I always have room in my bag to bring some extra pixie dust. Since we signed up too late for FE fun, I'm always up for some magic!