Welcome aboard, Mary!
Your grandson is going to have the best birthday aboard the Fantasy! I celebrated my last birthday aboard the Disney Wonder and it was certainly one to remember!
Though there are often Mickey ears available in the shops onboard the Fantasy, I can't guarantee which ones will be available during your cruise, and onboard, they do not embroider the names onto the hats like they can do in the parks. But, I have a few suggestions for you.
If you are planning on spending a few days at Walt Disney World before you set sail on the Fantasy, you'll find a few of the shops there, including The Chapeau at Magic Kingdom, will have birthday ear hats, and there they could put his name on the hat. You can see what they look like on Walt Disney World's
Customize Your Celebration page.
If you won't be enjoying the parks before you cruise, you might consider ordering
Mickey ears online from the Disney Store and adding personalization to those.
Also, look at the
birthday packages and cakes available to pre-order from Disney Cruise Line. Your stateroom could be decorated as a surprise when he enters!
One last thing: Be sure to note that it's your grandson's birthday on your cruise reservation. If you call your travel agent or Disney Cruise Line, they can do this for you. Then your servers will be aware and may surprise him on his big night with a Happy Birthday song!
Have fun planning your cruise, Mary!