Teddy Lupin had known Scorpius Malfoy and Albus Potter since they were born. He knew everything about them. He knew that there was no chance in hell that Scorp was going to be Sorted into Slytherin, and that Al's fears about being a Slytherin were completely unfounded. They were both natural Gryffindors.
He knew that Al used to brew Potions using his parents' old textbooks at the age of seven, and that he didn't like toothflossing stringmints but he ate them every time Hermione gave them to him just to make her happy, and that he secretly loved the Weasley sweaters, even though he complained about them along with the rest of the kids.
He knew that Scorp's favorite hiding spot in the Malfoy Manor was the crawlspace under the drawing room, and that he dreamed of being a Beater for the Chudley Cannons, and that his favorite day of the year was Christmas Eve, when the whole family came over and Teddy did a one-man Christmas pantomime.
Therefore, he knew the exact moment Scorpius fell in love with Albus, because the tone of his letters changed, and he was much quieter that summer. He knew that Al was in love with Scorpius before Al knew himself, because he spent the day after break chattering about him and everything they did together.
He knew what to do when Scorp hid from his family that Christmas, depressed because Al was still being thick-headed and didn't know anything. He knew that, the moment Scorp stepped into the Burrow on Christmas morning, Al fell in love.
He was the only one who wasn't surprised when Molly walked in on them snogging in the broom cupboard.
I know, I'm on a roll today. This is my first next-gen fic, but I love Teddy and I love AS/S, so this was mucho fun. :) Hope you like!
EDIT: BIG THANKS to CrystalPrison for noticing I'd picked Sweetheart instead of Surprise from the drop-down list!