Spoilers
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He's technically not his son, its his twin brothers son. He never had a son. It's really complicated and you have to continue reading to understand, but just know that he technically doesn't have a son. (No excuse for ignoring the little boy though).
Well he actually doesn't really ignore "his son", he determined the best course of action being for his son to go to the boarding school so he sent him. And he's getting weekly update on Damian's life so it's just his way of taking care of the boy.
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Thst means, if there is any chance that the two children got together, it is not considered incest (๑•ㅂ•)و✧
But then....among the "everyone in the duke's residence(?) he killed", his half sister might be one of them (=・ω・=)
I keep forgetting he already has a son and thats sad beause that means he doesn't care about his son so much we haven't even had a mention of him since the first couple chapters