There have been several threads on this over the years with good ideas.
I am always fond of recommending one of the commissioning gifts I received, a beautiful small wooden box with the Navy crest and my name and commissioning date, used for insignia, ribbons, pins, devices, etc. I attached a screenshot below of results of a Google search “Navy wooden keepsake box.” Mine looks like the LaHart one, with Navy crest, and a small brass plaque with my Ensign Name USN and commissioning date, but smaller, and with a hinged lid like the AF one shown. No doubt there are AF varieties, and many more styles/sizes. My three uncles who had been WWII battleship and destroyer sailors got it for me, and put a photo of the three of them in uniform together inside.
Try to feel out if he really wants the saber, as it may not be a required uniform item until a more senior rank. They are a royal pain to travel with. I wore my Navy Officer sword maybe 5-7 times in a 26 year career. Most people find themselves wearing them in wedding arches more than their Service ceremonies.
Coin racks are always nice. There will be many versions that can be customized and have service affiliation.
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