If we were in the middle of the Ariel story line, then I’d probably say Triton. But Ariel’s over, and when they did do her story, they rigged it so it didn’t involve any underwater interactions. They even rigged it so Ursula was represented above water in both the guises we saw her in. Before Ariel’s story started I’d wondered if we’d get under the sea scenes and how they might pull that off effects wise, so the fact that they avoided it makes me think it’s a thing they didn’t want to tackle. I dunno how you could do Triton’s story, especially if it’s gonna be linked to Ursula, without showing them interacting underwater.
All things considered, I’d say it’s more likely that this guy will be Aurora’s father.