I think if Elias raised Jon he wouldn’t necessarily put in the effort to keep the more mundane things private. If Jon was adopted his own sense of shame might (at first) cause him to try and respect his dad’s privacy anyway but eventually he would give in to curiosity. Which is to say, I think Jon should see his father peeing and it should be a formative sexual experience for him.
He sees Elias holding his cock in his hand to aim the stream of urine and wonders what it would feel like in his hand. What it would look like without Elias’ hand in the way. What it would look like erect. How it feels for his father, to hold his cock while he pees. Watches the heavy stream and imagines Elias aiming it at him instead, or perhaps even emptying it inside of him. Elias responding to the sensation of his son watching him, having those fantasies, beginning to grow hard. Jon staring openly as Elias begins to pump his cock, the stream ends but he keeps going and he’s making all these soft gasps and Jon is so, so wet, and then he sees cum spurting as his father ejaculates into the toilet and Jon scurries away, face red and cunt dripping.
He can’t stop thinking about it for weeks afterward. The pee streaming from Elias’ cock, how flushed and heavy it looked when Elias became hard. Wondering if his father enjoyed peeing, if it aroused him. Would he want to pee in front of Jon? On him? In him? Jon fixating on it, trying to find excuses to be near the bathroom whenever Elias uses it, watching his father pee again and again and again…