Most lines require pair of slacks(either black, navy, or khaki) and button up shirt for the casual nights, sometimes a tie is required. For the informal nights just add a sport coat and tie. Formal nights vary by line but can vary from a black or navy suit to black tie(tux and appropriate black tie type accessories, look it up and memorize black tie ettiquette...........will serve you well). I get that there are athletic guys out there who don't like to get dressed up, but lay it out for him as a chance to shine as if he were in a hot manhattan or los angeles 4 star resteraunt. Really all the dress codes are about are atmospher and the practicality of making sure nobody dresses inappropriately to dine. Btw, just to mention, the only places he should be wearing white is as a shirt, a pocket square, or as a white/ivory dinner jacket(though definitely not required on Royal Caribbean) this time of the year. These are just basic rules to go by.