He is your father
As for how the father and son reunion is going to play out I’m not sure, but keep in mind that he’s in a coma now. What if the reason for this coma is just to prolong this reunion? Surely if Finn was awake and capable he would’ve accompanied Rey in her journey to find Luke. Right now this is not an option, which made me think that perhaps Rey will return to Finn and the others with Luke and Luke will see Finn and immediately recognize Sana in him or feel through the force that Finn is his son. But if that’ll be the case why wouldn’t Leia have seen or sensed the connection before? Somehow it didn’t feel exactly right, so I kept thinking.
What if Kylo Ren knows?
When baby Finn fell into the hands of the first order it must’ve been known that he was the son of Luke Skywalker. When Kylo was influenced by Snoke to join the First Order he could’ve eventually learned from Snoke that Luke’s son, who was presumed dead, was actually alive and well. Brainwashed with no knowledge of his ancestry and training to become a stormtrooper. This could also explain why Ren seemed to be already keeping an eye on Finn, as he knew exactly who FN2187 was. During the battle between Ren and Finn, Ren seems to hold a bigger grudge against Finn than just him being a traitor and a lot of the dialogue is actually between Kylo Ren and Finn. Rey is even pushed away from the battle by Kylo Ren. He only battles her when Finn is knocked out and she force grabs the Skywalker lightsaber to confront him. It seems really plausible to me that Luke and Finn will be reunited as father and son through Kylo Ren.
Happy space jellyfish, professional monkey trainer (read: mother of two boys), silly fangirl with a tinfoil armor and secret Sith lord