You are right, I just read that on his wiki page. The whole bit about who is father is and all the potentials is interesting.

Sean Astin was born Sean Patrick Duke on February 25, 1971 in Santa Monica, California, the son of actress Patty Duke.[3][4][5] At the time, it was reported that entertainer Desi Arnaz, Jr. was his biological father, and that Astin was born to 23-year-old Duke, after a scandalous affair with Desi Arnaz Jr., who was only 17. Arnaz's mother, Lucille Ball, disapproved of the relationship. Duke, who as a teenager had played a chaste young lady on The Patty Duke Show, related, "It was a horror...I was little Patty Duke who came into the living rooms on Wednesday nights." When Michael Tell, a writer, music promoter and publisher of the newspaper The Las Vegas Israelite, came to sublet her apartment and offered to marry her as a way out of the scandal, Duke agreed. The marriage lasted only 13 days in 1970, and ended long before Astin was born.[6][7][8]

On August 5, 1972, Patty Duke married actor John Astin, after having been in a relationship with him for two years. When the wedding guests were invited to speak, 18-month-old Astin looked at John and cried, "Daddy!", to which the Episcopal priest performing the ceremony remarked, "Well, that about does it!"[9] Astin subsequently adopted Sean. In 1973, Duke gave birth to Astin's half-brother Mackenzie Astin, who also became an actor. Patty Duke and John Astin divorced in 1985.[10] Duke married Mike Pearce in 1986,[6] and they adopted a son, Kevin, in 1989.[10]

When he was 14, Duke told Astin that Arnaz was his father, and the two developed a relationship, but in his mid-20s Astin met a relative of Michael Tell who suggested they were related. Sean set out to find his own truth about his biological father, and underwent genetic tests that showed that Tell was Sean's father. Astin maintains close relationships with all three men, saying, "Desi Arnaz Jr. loves me, and I love him. We are so close...Science tells me...that he's not my biological father. Science tells me that Mike Tell is." Astin considers John his father, as John was the one who raised him. Astin is also close to his step-father, Mike Pearce, saying, "I can call any of them on the phone any time I want to," says Sean. "John, Desi, Mike or Papa Mike...my four dads."[6]