No idea what an Ensign box is as for the Sword...they don't use that much and would be more something that sits and just looks pretty..but they wouldn't be able to take it to all their commands or even have room for it.
A new Ensign needs a ribbion box/bag..something they can put all their Navy jeweral in so as not to loose it.
Check out personalizationmall.com. - military personalized keepsake boxes. I ordered one for my son for his subs decommissioning. It is nice quality - and I was pleased with it, but for something as special as graduation from OCS you may want something better. You can look at the comment from March 30 and see a picture of the actual box on the review. The service was great. I bought it for my son to put his medals and dolphins in. There may be some other ideas on that site also. Congratulations to your son (and you)!!