It is allowed but, remember that it needs to be in checked luggage on the plane, you cannot carry it on. When you get on the ship you can carry it on but storage is limited AND most of all, not everyone will appreciate him if he plays it. An out of the way place with nobody around or on your private balcony or in your room softly is the place to play it. No matter how good he is, the playing is not going to be appreciated in most places. Remember you have the professional musicians that are not going to want to compete with him and when he goes to a quite place the reason there are other people there is because they want a quite place, not somewhere someone is playing his guitar.