I'm sure there's a cleaner way to do this by simply inserting a son somewhere for him, but I've never really bothered with modding family trees so I'm not really too certain about that. What you could do though is go into descr_strat and changes his starting traits so that he starts out with every single fertility trait. You could also remove one or two starting generals if you want in order to make more room for more family members. This should pretty much guarantee that he has a child.
But like I said, there's probably a cleaner way to mod the starting family trees. Hopefully someone else will know.
Edit: Just looked into it a bit. I think this is how you mod the family trees. At the bottom of each faction's character entries in descr_strat, you will find one or more lines that start with the word "relative". Here is how they work according to
this:
relative father, mother, child1, child2, child3, child4, end
So for the Byzantine Emperor, this is his relative entry:
relative Theodoros Laskaris, Maria, Ireni Laskaris, Maria Laskaris, end
That's him, his wife, and his two daughters. So you'd simply enter a new family member into descr_strat and add him into the relative family tree there. Or at least I think that is how it should work.