okay everyone saying that eclipse is going to adopt solar flare as his fifth child and they're going to have a similar dynamic as the rest of his children-hear me out hEAR ME OUT-
Solar Flare knew Eclipse from Eclipse's second iteration. That was nearly two years ago. They were forced headmates for months. Solar Flare is probably on of the few people (second or arguably more than Lunar as well as Ruin) who actually understands the way Eclipse thinks.
Remember those interactions they had in that laboratory on the sun? Remember that last moment they shared before being blown apart in that alternate dimension? There's no way either of them forgot that. For Flare, that's probably one of the freshest memories he has of Eclipse. Being there and providing the comfort he could and assuring Eclipse that he understood his intentions and that he really could have been a better person if he'd wanted to be.
It's been two years. It's not the same Eclipse. That Eclipse had a star and a superiority complex and an inability to find a new purpose. This Eclipse has four children and a pet dog and a platonic co-parent friend and is quietly grieving over so many events that happened so quickly. And Solar Flare is such a stark reminder of the past for him. Someone who would have encouraged him to be better, someone who saw past the visions of grandiosity and found the person who just wanted to prove himself to be as capable as who he used to be. And he isn't that person anymore. It's been two years and two more iterations of himself and he is so tired of being what he used to be.
So I don't think he and Solar Flare would be a father-son dynamic. I think they're just two people on two opposite ends of time who crossed paths once and nodded in solemn understanding the second time. And this time, Eclipse is crossing paths, and Flare is walking with him.