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Yes, he will be able to drink if you sign the waiver form as you are sailing in Europe.
Make sure that you sign up for the meet&mingle party that they throw. You go to royalcaribbean.co.uk >contact us> FAQs > and then under the On Board section it tells you how. You will have to register (it is free). It is a private party, the Captain attended our last one, and it is an excellant way to meet and converse with your fellow travellers before hand, and you may find someone of the same age group for your son.