Douglas expressed hope that incarceration would give 31-year-old Cameron Douglas the time he needs to kick the drug habit that has plagued him since he was 13.
"He was going to be dead or somebody was going to kill him," Douglas said. "My son was a drug dealer, and he tried to kill himself for a while, and I can't condone his behavior."
Last month, a judge sentenced Cameron Douglas to five years for dealing methamphetamine from a trendy Manhattan hotel.
"It's going to take that amount of time for him to rebuild and start himself afresh," Douglas said.
Douglas said his son's spirit is good but added that the past year's imprisonment awaiting a sentence has been "really, really difficult."
"With all the mistakes and the disease that Cameron has, he is a great young man," Douglas said, "and I wouldn't be supporting him if I didn't feel that way."
The 65-year-old Oscar winner has accepted a major portion of the blame for his son's problems, describing himself as an absentee father during Cameron's youth too concerned with building his career.
Ask by "Today" host Matt Lauer what advice he'd give to other parents whose children might fall prey to drugs, Douglas said, "You have to catch it early. Your options once your children turn 18 are limited."