I believe that Carnival no longer has "formal" nights. They have now designated those evenings as "elegant nights". Years ago about 25% wore tuxes-----I think that it has now changed to 2%. Most men wear sports coats (our thumbnail picture is me in a "gasp" white sport coat AFTER LABOR DAY), but many just wear a shirt and tie. I am sure that he will be a GQ candidate in his outfit--just be sure to stop at MANY of the picture areas and pose, one may be THE picture for your cruise. Also (if he is anything like me) tell him to be sure the pants are able to expand by the end of the cruise. Enjoy!!!
In response to the poster who mentioned a tux rental place selling a complete set for $55, may I show my $25 silver/black velvet tux from a rental shop on Ebay---they even altered the sleeves and inseam! True, I had to use my own accessories-but luckily I had them. The HAL Maasdam is the only ship I've been on with an escalator.