He would let his son be born, butā¦he will not make him train to be a magus. At least if his son didn't show good talents. Then when his son is at the age where he can reproduce, he will marry him to Forest. Thus, getting grandchildren with the DNA of Forest and his son. He can live for 150 years, so he will probably wait for a couple of generations until the family is big enough and then he would swap in and harvest them all.
"Good thing you weren't stupid enough to make the mistake of killing them. I would have sent you to the other world right after them, and if it wasn't me, the Major General would have done it." The woman declared haughtily.