The man called police shortly after 10 a.m. Tuesday, saying he was suicidal, and asked police to meet him, said Cpl. Debbi Carlson, a Prince George's County police spokeswoman.
He gave no indication he had hurt anyone when police and firefighters met him a few minutes later and took him to a hospital for a psychiatric evaluation, said Mark Brady, a county fire spokesman.
The mother called police from the same address at about 10:30 to report that her son had attacked her and run off. "The officers and dispatchers put two and two together," Carlson said.
The woman was taken to a trauma center in Baltimore for what Brady called "a near amputation" of one arm.
Police did not release either person's name.
It was unclear whether the attack happened before the man called police or between the phone call and the time he was taken into custody, Carlson said.
Charges against the son will be determined after the psychiatric evaluation is complete, Brady said.