Hit By Pitch?

[JRSooner]

This past weekend the opposing team on one of my sons games pitched one of our batters inside and ended up hitting our batters hands while he is holding the bat then he dropped the bat. He didn't turn on it or make an attempt to swing but the ump said it was a foul ball because the hands are part of the bat. Our player lost a fingernail from this, so it definitely did hit his hand and the ump didn't argue that fact but he still said it was a foul ball. Was this the right call?