@Shadow
He was told my his sword mentor/master (Seran's mom) that his dad had returned to the village after he left and was having sexy time with mom (told during his match with her during the tournament and afterward).
Towards the end of chapter 42, Seran's mom the red orge notes that his mom is pregnant which would mean that around the time he left... Kail then cuts her off.
While Selaia did get pregnant at this time, his dad had been at the village the whole time. You can see that he bid farewell to Kail in Chapter 4 Page 26. Seran's mom didn't say his dad returned to the village after he left. Seran's mom said his father and his mother was doing hump hump to the ding dong bells right after he left the village. His father has always been in the village.
Also, his match with Seran where he didn't convert his mana to magic (Chapter 41) Kail's sword (holy sword? forget the midget's name) notes in Chapter 42 that the destructive magic he used is used by demons.
Him being able to use destructive magic was not because he is a demon. He was able to use it because he now has double the mana he used to have, as he brought over his mana from the previous world. As stated in Chapter 3, a person's mana is inherent and cannot be changed. His mana was double because there are two of him in one body fused during the transfer, which makes him have the same amount of mana as demons. Sildonia also explained this in Chapter 6 Page 25. Sildonia specifically informed Kail the possibility that he will be able to use high grade magic in page 28 of the same chapter. It's not because he's a half demon. In Chapter 41, it's stated that Kail wanted to use a magic spell, but he didn't have time to cast the spell, so what he did was to fire off mana without casting it, a technique he had never used before. It could be one of those non-elemental magic that demons happen to use but can be used by anyone if they can assume the theory. In Chapter 42, Sildonia wondered if Kail didn't notice that the magic was the magic demons used, but she didn't even consider that he's a demon, suggesting that it's a taboo magic that anyone can use, but is not used because it's associated with demons.
What I had remembered was it showed a part where he was fighting the demon king and the demon king wanted to talk and noted that he was his father but Kail didn't believe him and slew him anyway (just scrolled quickly through all 60 chapters but can't find the scene...).
There is no such scene. You've probably mixed things up with Star Wars or one of those manga spoof scenes. Ah wait, you mean Chapter 2 Page 28? Yeah no, that's the editor trying to make a Star Wars joke, ending up confusing English readers. Nothing in that whole chapter suggested the demon king was his father.
Regardless, it is possible (though not probable) that his father was the new demon king, since nothing so far suggested that he couldn't be. However, there was nothing so far that suggests he could be the demon king or even any average demon.