At this point, I'd be kind of let down if Mon-El turned out to be the Prince of Daxam. Viewers suspected that was his real identity from the first episode and now it's been too long to make the reveal, it would be kind of meh.

I've got a feeling that they're his parents and have been looking for him so he can take his rightful place as heir. Remember when that Dominator immediately recognized him as someone important and therefore allowed him to escape with the rest of the hostages? He wouldn't have otherwise let Mon-El go and would've killed him on the spot. I believe Mon-el lied about what really happened on Daxum to hide his identity.