Not quite. The 40mm Bofors is still in current use. The 2pdr pom-pom was a different, lower muzzle velocity, less effective weapon - still in use but obsolete by world war II (40mm upper - pom-pom lower) 
Good point. the old 40mm Bofors Pompom was widely used as antiaircraft round in WWII. Probably the propellant burned, but the warhead didn't go off (it is designed not to until a certain number of rotations after launch.)
Still, without a barrel around it, he is lucky it wasn't worse.
I think that National Guard armories have a progam that encourages ordinance to be reported, and they can arrange for someone to come and Xray it to see if it is a hazard.
Be safe! A finger is a poor trade for the coolness of a paperweight.
Not quite. The 40mm Bofors is still in current use. The 2pdr pom-pom was a different, lower muzzle velocity, less effective weapon - still in use but obsolete by world war II (40mm upper - pom-pom lower)