I'm 6'4" and the one and only time I had a problem with the length of the bed was my very first cruise on an old rust bucket... that is long since gone. And I still fell in love in with cruising
On almost all the mass market lines the beds, and bedding have been an area the have concentrated on upgrading the last couple of years. He should have no problem stretching out for a great sleep. Just don't make him take the upper bunk