The Ungrateful Protagonist
First, the protagonist was a foundling, and he knew full well that none of this originally belonged to him.
Second, all the kindness Tang Hao now showed the protagonist was because he felt guilty about the previous six years and about Ah Yin. Besides, given Tang Hao's personality, what do you think would happen if he found out Han Yu wasn't his biological son?
Would he let him go? Or would he leave him in Holy Soul Village to fend for himself?
I think the most likely possibility is that he would have him blaze a trail for Tang San. Next, in the original story, why didn't Tang Hao take Tang San with him to cultivate after his Martial Soul Awakening, instead of letting him cultivate at an academy on his own? I don't think the Clear Sky Sect would have turned away such a talented Soul Master with Twin Martial Souls.
With Tang Xiao as sect master and Tang Hao backing down, would the education Tang San received have been any worse than following Yu Xiaogang?
Coming back to the point, do the protagonist's current actions make him an ungrateful wretch? The original story said that Tang Hao didn't take good care of Tang San. Once Tang San was able to move around on his own, Tang Hao relied entirely on him to take care of him. He only made a little money forging farm tools, and he took most of it for himself to buy alcohol.
So now there are two children. Would Tang Hao do any better than in the original story? There's no problem with having Old Jack help look after them, right? After all, Old Jack was very kind in the original story.
Since he was already helping look after one, why not look after both? Tang Hao wouldn't take care of them anyway.
So when the protagonist and Tang San couldn't move around on their own, Old Jack was basically the one looking after them. No problem with that, right?
One more thing: the protagonist of this book isn't a saint or a gentleman. He simply wants to survive and grow stronger.
For example, if you were the protagonist, what would you do? Would you try to please Tang Hao? Curry favor with him and Tang San? Just hope that when your identity was exposed, the two of them would remember their old ties with you?
Or would you try every way possible to get the greatest benefits from Tang Hao? What would you do?
Don't forget what Tang Hao's personality is like, or what kind of person he is.