There was a story I wanted to write, where Ferb trades himself so that Phineas can spend one day with Francis. At the end of the day, another angel (not sure which, but the one that Ferb made a trade with) said that Phineas had to choose between his stepbrother or his father. He chose Ferb in the end, and Francis told him that he was proud that he did:
“It makes more sense that you would choose a boy you knew your whole life, than a man you knew for only two years.”
Francis then expresses his gratitude towards Lawrence, for raising his children, and then congratulated Ferb on the amazing things he’s done and for… well, tolerating Phineas. Then before he leaves, Phineas asks him if he could go see one last person for a minute; someone who really needed to see him one last time.
You can guess on who he went to see.