No, that's not a speculation. In the novel version of Boruto the Movie, Momoshiki is plainly called Kinshiki'son. At the same time, there has never been mentioned such a term as "royality Ootsutsuki". They're all the same Ootsutsuki, the only thing that ties some of them like Momoshiki and Kaguya together is their arrogant attitude towards humans whom they deem "inferior beings". Yet, it's not surprising for the Ootsutsuki who are highly advanced inter-dimensional alien race.
How could they is father and son ? If Kinshiki is Momoshiki father, he shouldn't call Momoshiki just like that he is his master or sensei. Momoshiki always called Kinshiki is : Kinshiki !. He never called him : Father or Papa. I think he only is Momoshiki partner.
Drectórios, please do not double/triple post. If your comment is the latest one, remember to click the edit button on it and add to it rather than make consecutive comments within short periods of time without waiting for others to respond.
We've already told you where the info is, now if you want to go actually look, that's on you. We all already understand they're father and son, but no one just remembers off-hand which page it is. The book is available to read at various places online, go for it.
"Congratulations, it's a boy!" :-D "Your boy will grow up to be big and strong, and then eventually you'll become food for him, just as your father became food for you." :-/ In any case, you've given food for thought (no pun intended).
Edit- Oh, I just thought of something. Wasn't it presented as voluntary in the movie, but not in the manga?
In Naruto Shippuden Ultimate Ninja Storm 4 Road To Boruto, it's stated that Kinshiki is Momoshiki's FATHER FIGURE. That might mean that he's his father, or just that Momoshiki considered him his father.