Hey all.
I got a theory about Soos's father.
I think Soos's father is Grunkle Stan.
I know it sounds far-fetched, but I do have some evidence.
First of all, we know Soos's father sends Soos postcards from all these different places around the U.S. We also know Stan has been around the U.S. You might be thinking "But wait, the timing of those two details don't match up." To which I tell you, Stan may have kept postcards from some of the cities he visited, and is trying to cover up where he is to Soos. This makes sense because Grunkle Stan is good at scamming people, and Soos didn't appear to even suspect his father was lying about where he was.
Another thing, Grunkle Stan hires Soos immediatly after he walks up to the Mystery Shack. This might imply he knows Soos in some way, or may be related to him.
Also, Stan and Soos are both voiced by Alex Hirsch, thus sounding strikingly similar to each other. This may be through genetics; a father could pass a certian trait to his son, that being, in this situation, his voice.
I don't think Soos knows he is the son of Stan.
This makes Soos Dipper and Mabel's 2nd cousin.