I thought I saw something that outright said or heavily implied that he was the son of Alessia and Morihaus. Now I can't find anything of the sort. So is it true?

Probably, he was the successor of Alessia. TIL's timeline says this:

"1E266 The Elder Council elects Belharza the Man-Bull as the second Emperor of Cyrodiil."

So, what did we learn today, kids?

Never elect a minotaur king of all Cyrod?

Yes, good kids.

The term Man-Bull and the fact he is the elected Emperor kind of gives away his relationship with the two. So I'd say yeah.

And that, children, is why minotaurs live in Cyrodiil.

So Alessia was a damned furry?

Worse than that, a pasipha?an.