So what if he actually cannot say it? Maybe there's a curse or something on him that prohibits him from directly divulging information?

I thought about that too, but the thing is, I was thinking Surya, like Helios, is able to travel within the dream realms. So even if he couldn't speak in real life, I think he would be able to do so while he's in the dream realm? The one thing that can make this work though, is that one of the antagonists I have in mind was a former guardian priest, and has all of the same ability as Helios and Surya, and the co-villain he works with is a sorceress-like character, who could cast spells. So if they work together, I think they might be able to put a curse on Surya, but the question remains: why would they do that? In my mind Surya tried to prevent them from going after Serenity and sealed them within a pocket dimension with himself. So if they were trying to disable him, I feel like trying to kill him rather than curse him would've been far more effective.

Understandable.

Maybe he's trying to see if he can solve it on his own? Just personal pride being a silencer? Doesn't want to have to tell them too much about himself? Isn't willing to share so much of his own past with these other people he doesn't know—perhaps is worried about getting to close to them, as he got to Serenity, and losing them as well?

Mmm, I can't really see that, to be honest. :\ If Helios is anything to go by, guardian priests are very protective of their charges, and would readily risk their own well being to protect their liege and their planet. If anything, I think it would make more sense for Surya to want to let them know what is going on asap. The only alternative I can think of is him trying to keep a low profile, least the enemy becomes aware that he has arrived on Earth. But still, that doesn't explain why he wouldn't at least give a warning to Neo Queen Serenity, given that he knows this enemy is capable of wiping out a whole star system. Maybe he could warn them, but withhold information about his own background? But I can't see why Chibi Usa or the Quartet wouldn't whittle it out of him, since they would have a hard time trusting him when they have no idea who or what he is.

You're making it awfully hard for him to not say anything. ;D

On a less-unhelpful note, would it be possible to literally interrupt him?

Developing Helios as a character is one of my goals for the sequel, and Surya will be one of two characters that will act as foils for Helios. We know Helios took his duty to protect Endymion and Elysion very seriously, and was willing to put his life on the line. His dedication to his duty would become a hindrance in the sequel story, as he becomes torn between his love for Chibi Usa and the knowledge that his role is to advise and guide her. His conflict is only further aggravated by the fact that Chibi Usa some times acts childishly (e.g. wanting to go to an amusement park with him after school when she should be studying for high school entrance exams), and Shou finds himself torn between wanting to be strict with her and making her happy by giving her what she wants. (I also want to mirror his struggle between his sense of duty and his heart in a subplot related to how his family expects him, as the only heir, to study business and take over the family company, when Shou would rather study classical literature and go into academia. But that's a whole other plot line that doesn't necessarily factor into this discussion.)

 
Surya’s role as a foil will be as someone who decided to follow his sense of duty rather than his heart. Like Helios, he takes his duties very seriously, and the fact that Rahu and its minions took him by surprise and invaded his home is a sore point for him. He tries to compensate for his past failure by sealing the dreamscape entrance that Rahu and his minions used previously to enter his planet. Little does he know that he’s already too late, and the enemy had already started to quietly make their move. It’s only after the first monsters appear that Surya realizes it’s too late (this would be the manga canon’s equivalent of when Chibi Usa reveals she’s from the future), and swallowing his pride, tells the others who he really is and why he is here. The difference I see with the manga is that instead of dragging this out for three chapters, the reveal of Surya’s identity will come quicker. Surya’s pride and his attempt to redeem his failure shows that blindly following any one path without considering the alternatives can have disastrous consequences.

I'm glad that you were able to come up with an idea that fits your story without feeling forced! I always enjoy a good foil in writing, and I think what you've proposed positions Surya and Shou nicely for this purpose.