Well for it to have any long lasting significance, i would put his full name on it, the date of his 21st and who it is from, it is a present that he should keep with him all his life and pass on to his own children/grandchildren, and so it should have a set place and time in his life to make it priceless to them.
I had Tempus Fugit put on the back of one for my Hubby it means time flies. The engraver said " oh yeah I always thought your other half was Greek is that his name" lol.