He's lovely when he sleeps. Oh, sure, so is Ferb, but as much as he likes the green haired boy, its not him Irving comes to watch.
No, that would be the young redhead, currently curled up in position number 3. He's watched him enough times to memorise the way he sleeps, to know when he's too cold and when he's too warm. Number three means he's content, if slightly warm, and Irving cracks open the window just a tad.
For Phineas' benefit, of course.
He keeps watching as he grows up, as the machines get more and more elaborate and he shoots up, never as tall as Ferb (6'1" compared to 6'7", he measured them, he always does) and shyly gives Isabella that wedding ring, from the shop that Irving laid out the advertising for on Phineas' work desk, because he knows how busy he is and after all, he's good at researching for him.
He doesn't get invited to the wedding.
He comes anyway, hiding away and watching. Ferb knows he's there, he can tell from the way his shoulders tense up, but he's not going to say anything.
Irving knows.
He always knows.
Just like he knows that Isabella was the one who made sure Phineas didn't invite him, because he gives her the creeps and she doesn't like the way he watches her fiancé.
Husband now. Irving notes it down. She doesn't understand, she doesn't love Phineas the way he does, doesn't love him enough to know everything about him and
She doesn't even know his sleeping patterns.
But Irving knows. And its such a satisfying ending, the hero (because Phineas is the hero, he's the main guy, he's the one who does all those great things Irving knows) getting the girl next door and doing these amazing things.
He stops leaving the house, except to check up on Phineas.
Albert knows. He understands how love works, he knows its healthy, and of course its healthy, otherwise why would his brother let him go on so long and why wouldn't phineas stop him and
The others, they don't know, don't understand, and they try to stop him after the initial acceptance of the creepy kid with an obsession has worn off. They try to keep him away from Phineas, destroying his cameras and calling the cops.
But its ok.
He knows.