The Spa facility encompasses several other facilities and is priced at around £80 per person for the duration of your cruise or pay on a daily basis. When you booked the cruise under the para special arrangements you should have told them then, but whoever booked the cruise for you contact them and get them to pass it on or using your booking ref contect NCL direct or through the Latitudes club membership section if you are a member. [Latitudes is a loyalty card more cruises you do more benefits you get].
Meant to say if you get this sorted then there will be a little celebration for your son on the appointed day. When on the ship check either with the restuarant manager or the maitre'd if they have been advised about your son's birthday. Usually a cake and a wish you a happy birthday from some of the waiters in the main diningroom.
Your son can also join the kids/teens crew and there to is another option to be able to see a party time. Joining is freee.
I joined the spa for the 2 weeks, its normally $150 but i managed to bargain the manager down to $300 for 3 of us! it is nearly always empty even on sea days and i found it well worth the money. It looks out over the front of the boat (you see the same on the bow cam on the TV) very relaxing and an escape from the heaving pool area!