Well, according to Wikipedia, here's the list of male Cassadines who could've been Sam's birth-father, and who are approximately Alexis' age or older. I included the deceased male Cassadines because they're. . . you know, Cassadines!
Anthony "Tony" Cassadine (deceased) - (Andre Landzaat, 1981) - Part of the Cassadine family. Former lover of Alexandria Quartermaine. He and Alexandria were frozen to death in 1981 when they unknowingly walked into Mikkos Cassadine's ice chamber.
Dimitri Cassadine - (Michael Carvin, 1986) - Mikkos, Victor and Tony's cousin. It was unclear if he was related to Petros, another cousin who bore a striking resemblance to Mikkos. Came to Port Charles from Brazil as the poor Cassadine, something no one was aware of, and established a business relationship with Sean Donely and the Quartermaines in February 1986. Helped Sean drive a wedge between Celia Quartermaine and Jimmy Lee Holt using Pilar. With the Donely/Cassadine merger now public knowledge, Monica began assisting them in bringing down the Quartermaines. After Jimmy Lee announced that the merger would make them richer, he and Edward Quartermaine decided to invest their fortune, unaware it was fake. With 50% of the Quartermaine fortune in his grasp, Dimitri betrayed Sean, who then tried to make up for his scheme by warning the Quartermaines. Unfortunately his warnings fall on deaf ears as Jimmy Lee and Edward sold the other 50% stock, thereby assuring their financial ruin. Having obtained the entire Quartermaine fortune, Dimitri fled to Brazil just as the Quartermaines were informed of their losses. It is unknown whether he is alive or deceased, as he has not been seen, heard, or spoken of since his departure in April 1986.
Mikkos Cassadine (deceased)
Petros Cassadine (deceased) - (John Colicos, 1984–85) - Cousin of Mikkos, Anthony, and Victor of the Cassadine family. Looked identical to Mikkos.
Stavros Cassadine - (John Martinuzzi, 1983; Robert Kelker-Kelly, 2001, 2002, 2003, 2013–) - Son of Mikkos Cassadine and Helena Cassadine of the Cassadine family. Father of Nikolas Cassadine and grandfather of Spencer Cassadine. Killed by Luke Spencer in 1981, but brought back from the dead by his mother Helena, who had him cryogenically frozen until his injuries could be healed. Supposedly died a second time after falling into a bottomless pit while being chased by Luke Spencer. Returned from the dead in 2013, and was revealed as the culprit behind Lulu Spencer-Falconeri's disappearance and his son Nikolas' shooting.
Stefan Cassadine (deceased) - (Stephen Nichols, 1996–2002, 2003) - Son of Mikkos Cassadine and Helena Cassadine of the Cassadine family. Uncle of Nikolas Cassadine, and raised him like a son. Ex-husband of Bobbie Spencer. Ex-fiancé of Katherine Bell. Ex-lover of Laura Spencer. Fell to his death from a cliff after being stabbed by Luke Spencer.
Valentin Cassadine - Son of Mikkos Cassadine of the Cassadine family Biologically born a Cassadine as a baby but was later removed. Poisoned Helena Cassadine, and wanted to seek revenge on her, Luke Spencer, and the other Cassadines and Spencers.
PS As an aside, I am amazed at how up-to-date Wikipedia is. They've already got the current Stavros (RKK) 2013 stats and plot line. Those people on the Wiki staff are BUSY!