Hagerstown police Capt. Paul Kifer says Jack Garcia died Sunday at Children's National Medical Center in Washington.
Kifer says police will confer Monday afternoon with Washington County prosecutors about bringing additional charges in the case.
The mother's boyfriend, 30-year-old Robert Wilson, was charged Wednesday with first-degree assault and child abuse. His public defender hasn't returned calls from The Associated Press.
The mother's brother told police the boy was handcuffed Tuesday as punishment for stealing the birthday cake. He says Wilson hit the boy in stomach after the handcuffs were removed.
It's not clear whose cake it was. The boy had had a birthday three days earlier.