"We were going to make everything legally binding in less than two weeks," Stern, 38, told The Insider and Entertainment Tonight this weekend in an interview that aired Monday night. "I can't even talk about it."
Stern's older sister, Bonnie, echoed his claim, telling the Associated Press, "They had plans to get a yacht and to buy an engagement ring. They were going to get married Feb. 27. It was going to be a real marriage."
Stern and Smith, who died on Thursday, exchanged vows in a non-binding commitment ceremony in September. Stern claims to be the father of Smith's 5-month-old daughter Dannielynn.
He also told ET that Smith left behind a will naming her late son Daniel as her heir. Daniel, 20, died in September three days after Dannielynn was born.
Stern vowed he'd fight for custody of Dannielynn – and had harsh words for Smith's mother, Virgie Arthur, who flew to the Bahamas last week because she "feared for her granddaughter's safety," the Nassau Guardian newspaper reported.
"As long as I have one breath left in my body, that woman will not see Dannielynn," he said. "Anna despised the woman, just despised that woman."
Stern will also have to fight photographer Larry Birkhead, who also claims to be Dannielynn's father and is planning to ask a Nassau court to permit a DNA test to determine the infant's paternity, his lawyer told PEOPLE Monday.
On Tuesday, a judge ordered that Danielynn must stay in the Bahamas until both the paternity issue is settled and Arthur's application for guardianship is decided.
The emergency order from a justice of the Bahamas Supreme Court effectively prevents Stern from taking the child out of Bahamas.
Birkhead, 34, had posted a tribute to "sweet Anna" on his Web site, but the message has since been removed. All that remains on the site is a photo of the couple on a beach and the words: "Anna Nicole Smith 1967-2007."
On Monday night, Jackie Hatten, a 16-year friend of Smith, told CNN's Larry King that Smith had told her Birkhead was indeed the father of the baby. Asked what relationship Smith had with Stern, Hatten replied: "He did her legal advice. He was the gofer. He would go get her food. He would go carry her bags."
On Tuesday, Stern's sister, Bonnie, told the AP that Smith was running a fever of 105 degrees and a nurse was "icing her down" on the day she died. She also said that Stern had tried to convince Smith to visit a doctor, but she refused.
In another development, the Hollywood, Fla., police department has announced that the 911 tapes from the day Smith died will be released Tuesday. Hollywood rescue responded to the Seminole Hard Rock Hotel and Casino after Smith was found unconscious in her hotel room. She was later pronounced dead at a hospital.