First Trimester by Sarah Havan

Riley Morgan went to the lake to ride out his heat alone, but due to his unusually high level of horniness, he sleeps with Endre, an exceedingly handsome, random guy he meets. When Endre’s knot begins to form, he realizes they aren’t just having sex, they’re mating.
Endre has a problem. He glossed over the obvious signs that Riley is an omega and ends up locked inside him. There’d be major repercussions if the world found out the Prince of Andolva was an alpha and impregnated an omega, so he has to bid him farewell. But after they part ways, he knows he made the wrong decision.
Now they can’t seem to find each other, and if Endre can’t locate Riley soon, they might never see each other again because The Omega Relocation Unit, a branch of the government that hunts down omegas and traps them in preserves, has been on patrol in Riley’s neighborhood. One misstep from him, and he’s taken away forever.