This prompt is both very sweet and very sad.
The main character is at least far enough apart in age to not suspect that his dad wasn’t his biological dad. It’s not stated if he was adopted at birth or if his biological parents kept him for a while and then decided they didn’t want to raise another child at their age. If he was adopted at birth, it seems highly unlikely that the gap between them is less than eighteen years, unless this was during a time period where adoption was less formal and he could have adopted him at a younger age. I could buy seventeen if their biological parents said, “Okay, we will keep him until you are eighteen and can legally adopt him, but you are in charge of him and you move out as soon as that happens.
While they could have other siblings, I kind of don’t see that. I see their biological parents as having had his dad in their teens, maybe even while they were underage themselves, but while in college is a possibility. They had him and felt pressured to get married or they married young and didn’t plan to have any kids. They decided to keep his dad and were looking forward to being child free when he grew up or had already started their child-free life.
I can see them raising him to make sure he would be self sufficient at eighteen so that he could move out as soon as he was legally able. If they could get him moved out sooner without looking bad, they probably would. If they have the money, they might have started him over financially secure with the skills to keep him that way or they might have made sure that he saved every penny he earned as a kid, getting him employed as soon as they legally could, maybe even entering him in cute baby contests before he was a year old if they thought he could win.
Then they got pregnant with child number two and they noped right out of that situation. They chose to have him, but long before he was born they were actively looking for someone to adopt him and the main character’s dad heard what they were doing and he wanted to keep him. In this imagined life he’s had, I don’t see him as having very much affection and I can see him wanting to give his little brother everything he never had as a child, the love and affection their parents never gave him and at the same time, he wants that for himself, so he convinces them to let him adopt his brother and he sets out to make sure both of them have a life full of love and happiness.
They may be in contact with their biological parents, but my guess is even if they are, it’s not a close relationship, though it’s not impossible that they are better as grandparents who see you once or twice a year than as actual parents. Do you think his dad kept in contact with their biological parents after adopting him?