Harry walked into his son's bedroom on August 28th, Matthew's birthday, carrying the letter admitting his son to Hogwarts. Harry knew Matt desperately wanted to go to Hogwarts and he to wanted his son to have this opportunity no matter how scared he was of Severus finding out the truth. He walked over and sat on the bed. He gently shook the child's shoulder and Matt woke up.
"Hey! Happy Birthday Big Boy! Here, here's your Hogwarts letter and your Aunt Poppy's coming to take you to Diagon Alley! No don't look at me like that Mattie you know I can't come with you, I don't want to risk seeing someone I know please try to understand sweetheart!"
"I do daddy but it doesn't mean I have to like it! I'll be down in a minute to go to Diagon Alley. Thanks for the Quidditch Balls by the way daddy!" Matthew smiled at his dad who gave him a gentle hug back before going back downstairs. Ten minutes later Matt and Poppy were flooing to Diagon Alley.
Severus was walking down Diagon Alley thinking about Harry. He missed his love dreadfully and was devastated that Harry had left him. He had also been furious to find that Sirius Black and James and Lily Potter were alive and in hiding until the war had ended. He was daydreaming about Harry still being there when he turned the corner to the Apothecary store and saw Poppy Pomphrey walking across the street with a young boy who looked like a mixture of Harry and himself. He decided to follow them and see if he could find out who the young man was. The pair crossed the street to Quality Quidditch Supplies.
"Come on Matt sweetie, you know your dad will kill me if I let you buy another broom. After all you already have two!"
"Oh Aunt Poppy please! I only have dad's old Firebolt and my original Nimbus 2016 that I've had for like 3 years; please can I get the new Firebolt Inferno! Please??"
"Oh very well but you can explain to your dad! God you have had me wrapped around your little finger since you were a baby!" She said before following her godson into the store. Severus was shocked by what he had heard and he realised that Matthew could probably be Harry's child. He apparated straight back to Hogsmeade and then flooed to Dumbledore's office at Hogwarts where he demanded to see a list of the incoming 1st years and there on it was Matthew Severus Potter-Snape!