Maybe not as a son, but as a little brother (Yes, Percival has protested repeatedly that he is older, Jacob usually just pushes a pastry into his hands and pats him on the back before ignoring it entirely).

The man needs someone to look out for him, and he clearly isn't interested in getting himself a partner of any sort. So a big brother to get him taking his potions and eating properly, dragging him away from work on the weekend, even if it is just to the sofa with a book, it was necessary really. Jacob had no choice.