The Father: Alternative Answers
So obviously, the fandom consensus is that Jesse or Sid woud have eventually been revealed as the Father. Sid seems to be the community favorite, but Jesse still has a lot of supporters, and every other option seems less likely.
But just for fun, let's come up with some alternative answers. Not ones that you think are likely necessarily, but ones that could have be interesting, unexpected, and/or cute to see.
I'll start with my selection of Charlie as the Father. With the added twist that he, Sophie, and Valentina are all together as a throuple.
Now obviously there are some major flaws with the suggestion, such as future Sophie repeatedly calling Valentina "Aunt Valentina" when talking to the son (rather than your other mother or something like that), but it also works surprisingly well in some ways.
First up, Charlie is included in the montage of possible Fathers at the end of the pilot, but there's really zero indication that he is ever really even remotely a contender to become the Father. This would explain why he's still in the list and be a geniuine suprise twist (as long as it was done well).
Secondly, at one point Sophie tells the son that by the time she was forty she had kissed all the people in the friend group and the son reacts with shock that "even Aunt Ellen" is included. Which implies that was the biggest surprise for him, rather than for her to have kissed Charlie or Valentina. If they are all together, then obviously he wouldn't be surprised about that.
And my favorite thing it does is make the season 2 finale turned series finale final reveal stronger. Because Sophie isn't just revealing how Valentina and Drew dating lead to Charlie and Valentina having a kid together (or raising one together), but that would mean the baby Alex reveal would be talking about the son's older half-brother. Which would be more meaningful to him, but also to the audience if that is the case.