beta
My first thought was it's secretly mpreg genre, until the short story saying it's not mpreg nor omegaverse lol.
My best bet is that it's actually his sister's son, but his sister didn't have a husband and she died so he took in her son like how it typically goes lol
Or maybe it's a real plot twist where it's really genuinely his son and he actually really did marry and have kid with a woman..