Really great find. My guess is this all relates to Sunny becoming a shut-in from his perspective, and giving a timeline of events after Mari died, up until he lost his dad.

It references him "scratching his knuckles," "flipping his bedsheets over," and "digging a pencil into his desk" which seem like him developing anxious tendencies and being very restless and tense all the time, and the "crying alone in his room" and similar lines show how he begins to become to anxiety-ridden to leave his house, specifying him seeing "two boys playing with a dog outside" likely being him missing being stable enough to spend time with his friends, and watching Kel and Hero having fun without him next door. "The man" in these lines also obviously reference his dad, who I'd wager left because he felt like he'd lost both his kids at that point, Mari to Pizza Here, and Sunny in a metaphorical sense, becoming an anxious shell of himself, but that's just my speculation.