He meant like, He want's to have a wedding with just the two of them. (A private wedding)
Because Ei is originally his love, the child he have is something from his past that he failed to keep (he loves Rio so don't worry). The past like Ei having a sexual relationship with Angelo, and Yuujin marrying and having a child.
This resulted from their "obligations" and because of that they failed to show their affection towards each other.
That being said, Yuujin wants to have some lovey dovey time with Ei, without being binded from their past insecurities and past life decision failures.
That is also why Ei said "I won't apologize, so don't apologize".
Because without those decisions, they wouldn't end up like this in the present.
I agree with everyone else. He's not been neglectful. Jin has prioritized everyone and everything else over Ei; and his own happiness his family legacy and then Rio, while Ei has pretty much dedicated himself to Jin all his life By telling him he just needs Ei at their wedding, Jin is prioritizing Ei and their relationship for the first time ever. It's a gesture of commitment.
This dude just said he doesnt need his child to be in their wedding where tf ur priorities at? i get that theyre so in love and that they dont need anything else, but dude even ur own child??