Authorities arrested him and David St. Onge Jr., 20, on July 2 after finding the body of 88-year-old Jack St. Onge in tarpaulins in a wooded area in Schuylkill County.
The three men had been living together at the elder St. Onge's house in Plymouth Meeting since the winter, court records said. The killing allegedly occurred on June 10 after Jack St. Onge told his son he could no longer live there.
In his interview with detectives, St. Onge Sr. allegedly acknowledged he had a heroin problem.
Prosecutors on Monday also announced new charges against the pair, saying they used the victim's credit cards to buy more than $14,235 in gas, food, and other items.
St. Onge Sr. and his son were charged with one count each of theft, conspiracy to commit theft, access device fraud, and conspiracy to commit access device fraud, all third-degree felonies. For St. Onge Sr., those charges are in addition to third-degree murder, abuse of a corpse, and other offenses.
His son is awaiting trial on five misdemeanor counts, including abuse of a corpse, tampering with evidence, and unsworn falsification to authorities.
Both are being held at the Montgomery County Correctional Facility.