Thinking on it some more. If it will be left at "home" while he is in another state/country, no one will know or care as long as no family member goes shooting their mouth off to a Karen. IF the marines let him have it at all, I will probably be stored in the Arms room (1). By the time he is senior enough to live off base, he will be old enough to own it (2).
(1) I was Army, so Marines might be different. But, no enlisted living in the barracks cold keep a gun or major component of one or a fixed blade knife with him. They had to be locked up in the unit arms room. The same applied to Officers living in the Bachelor Officers Quarters. On some post it also applied to married officers/enlisted living in post housing.
(2) Depending on where he is stationed, he might be old enough to own it now.