Well, both sentences have different nuances meanings, in spoken German; here the one from Tifoso:So, Tifoso, in spoken Austrian German, one wouldn't say "Ich habe gewusst, dass sein Vater getrunken hat"?
The mere fact that one begins the sentence with "Ich habe gewusst" lets the speaker know that they're talking about the past?
- Ich habe gewusst, dass sein Vater trinkt.
Here everyone would think that his father is well and alive, and that the "Ich-Erzähler" knew that all the time (or for a long time at least).
And then the other one:
- Ich habe gewusst, dass sein Vater getrunken hat.
Now here another interpretation is more likely: the one that the father is already dead (without indicating whatsoever whether his drinking habit had anything to do with it or not), and that the "Ich-Erzähler" knew that all the time; or alternatively that his father is alive and well but that he's clean now, and no longer drinking.
So this sentence indicates that the drinking habit is terminated (not continued, something of the past), while the former suggests that the habit hasn't been interrupted.
Actually, I do not even know exactly how those nuances could or should be given in standard German - I guess that in Germany "trinkt" would be rendered with Perfekt ("getrunken hat"), and that "getrunken hat" should be "trank": but I could be wrong.