51-y.o. fairly fit male here. In the last couple years I've twice slept on a Carnival sofa bed made up for one, and I'll warn you in advance it's NOT comfortable - hard as a rock, and I like a firm mattress. Granted, neither were in the newer Mardi Gras cabin style so theoretically it could be less unpleasant if the couches are significantly different from the ships I sailed. Your son is half my age so he may fare better.
That said, I'd do it again if the cabin price is right 😛