The Prince had no idea what Corin had SAID in his letter to his parents. How much of Riku's situation he'd written about, when he might not have fully UNDERSTOOD his situation even. So he couldn't predict what they'd say.
Sora worry was that...they were wonderful. They were wonderful and they missed Riku and loved him and wanted him to come home right away. Whatever the price was they'd pay it. That they couldn't wait to see him home and back in the family and Sora couldn't say no. Couldn't say stay here with me, with no engagement and the promise of marriage from ONLY him.
The Prince rubbed his back a little before pushing Orion's nose away. "Do....you want to go spend some time with Corin?" Not that Sora WANTED that because then he wasn't there to hear the letter and what if Riku got emotional and he'd just be sitting here in the stables WORRYING. But he felt like if he let them do their thing that meant he was letting go enough that Riku would come back to him?
HE WAS JUST TRYING TO BE A GOOD PERSON OKAY? Orion nudged at his head because now HE was stressed.