Were the flashbacks to Daniel and H.W. after the "Bastard From A Basket" scene meant to show tender moments? I thought they were quite malicious, especially the second scene, where H.W. didn't seem to take it as teasing. The first, Daniel seems to be angry that he was toying with him.
It's a bit ambiguous (like a lot of this film) because there's no sound besides the music, but I kind of agree. The first scene ends with Daniel roughly pulling H. W. over his knee (presumably to spank him), and in the second, Daniel roughly smacks him in the side of the head (a little too hard be simple teasing).
It looked to me like he was just playing rough with him. I really thought all signs throughout the movie indicated that Daniel loved H.W. but was frustrated that he lost his hearing and took it out on H.W. without really thinking about what he was doing. (Look at the scene where he comes back from the school he was sent to. Daniel hugs him twice, holds his hand and tells him he loves him, even though he knows H.W. is deaf.) So when he told his father he was starting his own oil business, I think that Daniel felt, in his fractured state of mind, like he had no one else in humanity that he could depend on. Another thing to pay attention to is the scene Daniel leaves H.W. on the train to the school for the deaf (or whatever it was). To me, that seemed to be the point where he started to lose what little morality and restraint he originally had. But this is just a theory.
I had a similar interpretation to the one above. When we see H.W. in the future he is restrained and stoic in the face of Daniel's provocations. I got the impression that he learned the technique of just sitting back and watching his father's behaviour, knowing that he was quite erratic and sometimes scary. In the flashback he endures his dad's roughhousing because he knows that's what he does.
We could argue that they are about as tender as it is possible for a man like Daniel Plainview to be. The man is hardly an exemplar of warmth and human kindness throughout the film, after all.
Indeed, and though this might be a more YMMV topic, I suspect that Daniel knew what he was doing the whole time when "roughhousing." He's a smart enough misanthrope to know how to indulge his angry tendencies without going too far, disguising punishment as normal relations to everyone but him and his target.