i dont know the MA law, but a lot of legislation as it relates to magazine capacity, etc usually applies to the manufacturing of it. Like back in the 90s when they passed the "assault weapons ban", it went after magazine capacity higher than 10 rounds but my father owned lots of 30 round magazines and those were all grandfathered.. it wasnt illegal for him to own them, or even buy them at like a gun show etc, but it was illegal to manufacture them
there was a major supreme court ruling recently that some are arguing could make all gun legislation unconstitutional, so i certainly wouldnt use Taxachussetts as a measuring stick for anything..
anyways, im really sorry that this young man isn't going to prison... i know how upset that must make you feel