[I am a spoiler!!]1) Tidus is not actual Tidus, he's a dream of the Faythe. 2) At the start of the game we are actually "fake Zanarkand" which exists in a dreamworld and is maintained by the Faythe. 3) In X-2 we find out, more or less, about Shyuin who looks just like Tidus, and actually lived in real Zanarkand at a time that could have made him actual Jecht's actual son. 4> During the Shuyin encounters in FFX-2 Jecht's abusive dialogue can be heard in the background. You hear him saying stuff like "you're worthless, etc" This is in a fight against Shuyin, when Yuna, Rikku and Payne are your active characters.

This is where fan theories and ambiguity comes in.

Since Tidus was just a dream of the Faythe, in modern day dream Zanarkand, how could he actusllt be Jecht's son.

Since Shuyin was actually alive in Zanarkand, and looks like Tidus, and because we hear Jecht's abusing him....the accepted theory is that Shuyin was Jecht's real son and Tidus was just a Faythe created, dream based, distillation of him.

Jecht/Sin's reaction to Tidus makes more sense under those circumstances tbh....it's his son...,but it's not his son......(/spoiler)