"I'm offering to share my cat with you, and you can't even share the sun with me when I've got ice above my dorm? I'll just have to make an even bigger sun, then, just to show you." Regulus was, of course, only joking with him. Even if there was a way to make your own, personal sun, he didn't really want to keep one around. In practice, it might not be the safest thing to keep under your bed, however just to talk about like they were kids playing at make believe was something different. "And I'll also make my own moon to go with it. I don't know what I'd do with it, but I'd make it."
Regulus had no intention to tell anyone about the billywigs. Even as off his head as he was when Sirius told him about them, he realized his brother had charmed it private to him for a reason. Besides, everyone had secrets to keep from that weekend, whether they were their own or someone else's. "Sirius, I think pretty much everyone either said or did things they wouldn't have otherwise, and pretty much everyone I know has decided to keep quiet about anything they heard or saw. Why should it be any different for you? Besides, I don't want you expelled. Things would be far too quiet without you here."