Went on Leg End two years ago my son 16 used the rock climbing wall and loved it the mini golf is good, he didn't use the club but friends of ours boys loved the club and went there every night.
The youth team usually organise vollyball and basketball type games for the teens in the day. Quizes and card games, Dodgeball and Pool parties It can depend on the time of year as to how much they put on for the teens - Late teens do not have many organised activites in the day, although there may in your case as you are looking at the height of the season. You dont actually say what age teen - as they are split into two groups (Early teens and late teens). The older teens generally like to just hang around together and do their own thing - they have their own dedicated area to chill but they do organise things for them in the evenings. I am going on Legend in May with a 13 year old so I cant really give you a more definative answer until then.
We took my step daughter who is 15yrs, yes there is table tennis, swimming pool an arcade, she was quite happy with the rock climbing and the mini golf, she never joined in the teens clubs, she just went about the boat and did her own thing, even though her older brother was with us they did their own thing. We only had a total of 1 and half days on the boat, so she didn't get chance to get bored, and she made the most of the holiday. she has written a review about her holiday, your son might like to read it even though it might be different countries it might give him a flavour. www.CRUISE.co.uk/cruise-reviews/read_review_9578.
Hi my family and I have just come back from a 27 night cruise last Dec08 my kids are 11 and 13. My daughter cried when we had to disenbark as she had such a great time. There are many organised activities including table tennis matches, many organised pool activities including pool volleyball against staff members, pool parties. treasure hunts, pizza evenings, pyjama parties, movie evenings, in fact we hardly ever saw the kids until we insisted they return for meals. Your son won't be bored. Hope you all have a safe and happy trip.