I think the MC is the last surviving member of a forgotten family and he has an unknown potential that could take the deluca family to a status never thought possible
They already are the top family, whatever other families think about them. He's not an advantage for the DeLuca, he's a threat for some of the other families.
And I doubt that he's the last surviving member of a forgotten family. With his mother linked to the four original serpents, and his best friend being the heir of another family (he's the heir, not just the son of the boss, right ?) he would have been found and killed way before. What also demonstrate that he's not an advantage for a family ; he would have been integrated into his best friend's family if it was the case. At most he's an advantage that can benefit only to the DeLuca, but being who they are, they don't need it.
Anyway, if it's a forgotten family, then it's also a powerless family. His assassin (don't remember her name) is part of a disgraced family and have no power at all. So, the power of the last member of a forgotten family... It's just the power of a name, but a name without soldier is nothing when you're part of the mafia.
Cordia
gently asked him to join enrolled him by force because of, at least, his mother ; the least she can do is to protect him. And it's then, and only then, that the problems started. While being probably well known from all families (still because of who his best friend is), he lived a peaceful life before joining the DeLuca.
But no one would have left them the possibility to gain an advantage. The first one to discover it would have moved immediately, either enrolling him, or killing him. This while letting a possible threat in peace is more plausible. You do it in order to not draw too much attention to it ; perhaps are you the only one to show it, if you miss, the other will then see what you shown and try to use it against you.
The most important point rely on MC's best friend, and how his father acted with him. As cartoonish as the story is,
HopesGaming is far to be an idiot. Having the boss of a mafia family freely talk about his job (not the details of course) to a stranger isn't an error he would make. And we know that he did it. MC isn't at all surprised when both Luna and Gracie tell him who his best friend is ; he know it perfectly well.
Therefore, the said father know, at least more or less, who MC is. And it's probably not just because he's his son's best friend that he treated him like a son, that he taught him how to be streets smart. Yet, once again, despite doing all this, despite raising him to be a competent member of a family, he haven't tried to make him join his family.
Due to the bound between the MC and his son, it would be a really great addition to the family, even if the MC was no one. Having someone that you can fully trust, and that can understand you without the need to talk, it's the dream of any boss ; offering this opportunity to your heir, is the second best dream of any boss.
Was he asked to not do it, or is it because he assumed (probably rightly) that Cordia would be really pissed off ? This I don't know. But once again, it mean that, if it's an advantage and not a threat, then it's an advantage that can only benefit to the DeLuca.
He's not some lambda dude, but not important the way you think. And if he's really an advantage, then it's for Cordia's utopia, not for Cordia's family.