Police found Daniel Kerrigan, 70, unconscious in the kitchen of the family's Stoneham, Massachusetts home on Sunday (24Jan10) after receiving an emergency call reporting a "sudden death".
Mark Kerrigan, 45, was arrested amid allegations he attacked his dad following an argument.
He appeared in Woburn District Court on Monday (26Jan10) and was arraigned on suspicion of assault and battery on an elderly person and causing serious bodily injury.
Mark, who previously served in the U.S. Army, pleaded his innocence, and his bail was set at $10,000 (£6,250).
Kerrigan had been living in the basement of his parents' home after serving time in jail for assault and battery-related charges.
Daniel Kerrigan's cause of death is unknown, pending autopsy results.