Let them know. They will sing to him at dinner (even if there are others - they will sing for everyone...one night on my Feb Magic cruise it got eyerollingly comical the sheer number there were just in one MDR on one night - we quit counting at 12) and bring him a little chocolate mousse-type "cake". You can also get a button from Guest Services.
If you want more than that (stateroom decorations, a legit cake, etc.) you can order if from Gifts and Amenities.