Did the Army or Marine Corps give it to him?
At the end of WWII I believe officers were allowed to buy their semi-auto weapons like the M1 Carbine and M1 Garand for a nominal price. Full autos like Thompsons and BARs were verboten.
After the war the military surplussed a bunch of them and distributed them to the general public through mechanisms like the Civilian Marksmanship Program (CMP), again for a nominal price. Something like $5 per (which was a lot more then than now)